"Blair Waldorf, who is basically everything I hate about the Upper East Side distilled into one 95-pound, doe-eyed, bonmot-tossing, label-whoring package of girly evil."
-Dan on Blair in Bad News Blair
Blair and Dan begin the series as strangers before becoming enemies, but eventually develop friendship. Unlike most of the friendships on the show, their friendship is based on a mutual interest in culture; including literature, movies, and art. In season five, they enter into a short-lived relationship which ends as Blair ultimately chooses to be with Chuck. By the end of the series, they are no longer friends. However, Blair manages to forgive him by the five years flash-forward.
Blair and Dan start out as enemies in the first season. They meet in The Wild Brunch, after Jenny Humphrey reveals Dan is her brother and that he went out with Serena van der Woodsen the night before. Blair tells Dan that Serena slept with Nate Archibald, then ran away and lied about it. After this, their only interactions are usually when Serena is involved. They get on slightly better terms after Dan talks to Blair after she is humiliated by her mother and Serena (Bad News Blair). Blair and Dan work together in the finale Much 'I Do' About Nothing, in order to take down Georgina Sparks, who has returned and started blackmailing Serena.
In the second season, they still stick to interacting only when Serena is involved. However, Dan begins to dislike Blair more when he learns she started the rumor he was sleeping with Rachel Carr in Carrnal Knowledge. When Blair is rejected from Yale due to an anonymous tip that she hazed a teacher, she blames Dan for tipping them off, but he later tells her it was Rachel (The Age of Dissonance). Later, Serena brings Blair to Dan to talk her into telling Chuck Bass how she really feels about him. He convinces her to take a chance and do it, until Vanessa Abrams admits to him that Blair and Chuck recently used her as a pawn in one of their games. Angry, Dan instills doubt in Blair, who then humiliates herself in front of Chuck (Pret-a-Poor-J).
Dan and Blair begin attending NYU and start becoming friendlier. Still wanting to be queen but feeling she's losing to Georgina, she asks Dan to take her to Georgina's rooftop party. He does, but Blair embarrasses Georgina and invites everyone to go to a bar with her. Over Blair's games, Dan announces it was a set up to get Blair more popularity and tells everyone to stay and party with him, which they do (The Freshmen). In The Last Days of Disco Stick, Blair stars in a musical production written by Dan for the Tisch kids. However, she has to play referee between Vanessa and Olivia Burke, after they start behaving jealously toward each other after a threesome with Dan. Blair and Dan again team up to save Rufus Humphrey and Lily van der Woodsen's marriage after William van der Woodsen arrives in town to ruin it (Ex-Husbands and Wives).
After Serena is drugged, Blair and Dan work together to find Juliet Sharp and take her down, as well as find out what her motives are for destroying Serena's life. They ask Gossip Girl, who provides them with an address in Connecticut. They go together, and find Damien Dalgaard, who accompanies them on their mission for the rest of the day in The Townie. During Winter Break, Serena, Nate, and Chuck are all busy, leaving Dan and Blair the only ones still in town. They find they have similar interests in movies and museums, and begin to secretly hang out. In The Kids Are Not Alright, Dan calls Blair a dictator of taste when it comes to fashion, which makes her realize that maybe her future lies in working for a fashion magazine. Blair gets an internship at W Magazine, but is taken aback when Dan also secures one there. They begin to compete, and eventually make a bet that whoever can bring the best author to the W Party that night can stay at the magazine while the other resigns. Blair calls her author, and Dan calls his, and both can attend. While at work, Blair is given the chance to sabotage Dan but chooses not to. She realizes this is the wrong choice after she discovers he sabotaged her. The two get into a physical brawl and both are fired. While packing up their desks, Blair admits that she worked hard to get an internship while Dan only had Lily make some calls. After finding this out, Dan resigns so Blair can keep her job (Damien Darko). On Valentine's Day, Dan and Blair watch Rosemary's Baby together while on the phone (It-Girl Happened One Night). After Blair quits W Magazine due to it being too stressful, she goes to see Dan at the loft and they order pizza and watch a movie together (While You Weren't Sleeping). In Empire of the Son, Dan and Blair plan to reveal to everyone that they've started hanging out. However, their news is overshadowed by Lily turning herself into the police after forging Serena's signature. Rufus and Dorota Kishlovsky both think that maybe a budding romance is what's going on, though they both deny it. At the end of the episode, they share a kiss to see if there really are any feelings between them.
In The Kids Stay in the Picture, Dan confesses to Eric van der Woodsen that he might have a crush on Blair. Chuck, having heard Blair had a life changing kiss, eventually finds out it was Dan. He sets him up so Blair can see Dan will never be a part of their world, but Blair tells him that she already knew that, and the kiss was only life changing because it made her see that her future lay with Chuck. She says the kiss meant less than nothing to her, and Dan agrees to save face. Vanessa overhears and leaves Serena a voicemail telling her. However, Serena doesn't believe her. But just to make sure, she makes her newly arrived cousin Charlie Rhodes (Ivy Dickens), follow Blair on a secret date just to make sure. She sees Dan and Blair together, and tells Serena. But when Serena arrives, she sees it's actually Louis Grimaldi and tells Charlie she was probably just confused on who Dan was. Louis is there secretly, and when his family finds out and orders him to come back to Monaco, Blair devises a plan to keep him there. She acts like she's really in a relationship with Dan and Louis has just come to attend the Pink Party. At the party, she kisses Dan and has the picture posted on Gossip Girl. A jealous Serena realizes Vanessa was telling the truth and informs Blair that the only interest she has in Dan stems from the fact that Serena had him first. Blair, however, defends her friendship with Dan, and that she considers them intellectual equals and they have a friendship on a different level than either have with Serena (Petty in Pink).
In the finale The Wrong Goodbye, a newly engaged Blair leaves to spend the summer in Monaco with Louis. Before she goes, she tells Dan that she copied his Netflix queue and their movie discourse begins the following Monday. Eric notices the chemistry brewing between them, but says nothing.
In Yes, Then Zero, Blair goes to Dan when she's thinking about leaving Louis but becomes angry when she realizes not only was Dan going to let her leave everything with no questions, but he knew Louis was protecting her. In Beauty and the Feast, Dan is the first person Blair tells about her pregnancy voluntarily. She goes to see him that night, and admits she isn't sure if Louis or Chuck is the father, and Dan promises to be there for her no matter what happens. Eventually with the coaxing of Dan and Dorota, Blair decides to take a DNA test to figure out who the father is. But once she receives the results, she decides not to look. After Dorota finds out, she calls Dan, who tries everything to make Blair look. Eventually he physically puts them in her hand and encourages her to look, if not for herself, for the baby, and she does (The Jewel of Denial). After Dan's book Inside comes out, Louis reads the parts with Blair and sees that Dan wrote about having sex with her. Upset, he cancels dinner with Blair and their parents and lies that his mother was delayed. Blair realizes it was Dan's book, and she too discovers what he wrote. She confronts him at his book release party, saying that what he wrote could end her engagement. He apologizes, and she makes up with Louis later (Memoirs of an Invisible Dan). Soon after, Blair decides not to invite Dan or Chuck to her wedding shower. Upset at being left out, Dan gets drunk and shows up anyway, ready to reveal his true feelings to Blair. Chuck goes to get him and stop him from ruining Blair's night, but she becomes angry that he crashed the event. Afterwards, Serena explains to Blair that Chuck was doing her a favor by stopping Dan from ruining the party (All the Pretty Sources). After she decides to take a break from Louis, Blair goes to stay with Dan to avoid the paparazzi. She considers leaving Louis for Chuck, but becomes devastated when Chuck tells her to stay with him. Dan, seeing how heartbroken Blair is, decides to bring them together secretly to finally be together. After it's done, he tells Serena that he was going to tell her how he felt, but realized helping her be with Chuck was the right thing to do (Riding in Town Cars With Boys).
After Blair recovers from her accident and from losing her baby, Dan supports her by helping her secretly go to church and heal spiritually. However, Chuck and Louis suspect she's having an affair with Dan and plan to out them at Nate's New Year's Eve party. After Serena learns the real truth about why Blair is avoiding Chuck, she lies that she's back with Dan to save Blair from public embarrassment and losing Louis (The End of the Affair?). After Dan leaks the video of Blair and Chuck confession (and blames Serena for that) during the ceremony and Blair realizes Louis no longer loves her, she turns to Dan to help her escape the wedding (G.G.). He brings her to the airport to help her escape the country to get a divorce, and then takes her to a hotel to wait for the next flight. After Blair decides to stay married as to not ruin her family's future, Dan promises to be there for her (The Backup Dan). When Blair returns from her honeymoon, she decides to bring Dan and Serena back together for Valentine's Day. However, her plan backfires when Dan kisses her and she surprises herself by kissing back. Despite her feelings, she tells him that they can't be together because of the prenup and the dowry (Crazy Cupid Love). In Cross Rhodes, Blair kisses Dan and Serena unfortunately witnesses. At the hospital, they finally realize their true feelings for each other and enter into a relationship (The Princess Dowry).
Soon after, they attempt to consummate their relationship but are disappointed when the first time is awkward and terrible. After they both get drunk, they hook up in the elevator in the VDW building and decide that they connect "sexually" as well as intellectually (Con Heir). Blair receives her divorce papers, but doesn't sign them. Dan finds out that Chuck paid her dowry and assumes that that's why she didn't sign. However, she admits to him that she didn't sign because she wasn't ready to stop being a princess yet. To solve her problem, he dresses her up in a gown and brings her to the Met steps to meet some fans of hers (It-Girl Interrupted). In Salon of the Dead, they clash over who's world they want to let their relationship live in. To reach a consensus, they host a salon that ends up getting crashed by Serena, Nate, Chuck, and Lola Rhodes, Serena's cousin. It ends up failing, but they decide to try and make things work anyway. After reading that Dan is highbrow while she is lowbrow, Blair plots to make herself more relevant. In her quest, she ruins a party in Dan's honor but he forgives her, and agrees to help her find out who she really is (Despicable B). In Raiders of the Lost Art, Dan is offered a fellowship in Rome, Italy for the whole summer. He invites Blair to come with him, but she dances around giving him an answer. After finding out she's back to scheming with Chuck, he lies to her that he lost the fellowship to someone else. However, she finds out that isn't true and encourages him to take it. She agrees to go with him to Italy, despite Dan wondering why she hasn't said she loves him back yet (The Fugitives). In the finale The Return of the Ring, pages from Blair's diary are released, in those pages Blair admits she is afraid of her never being able to love someone as much as she loves Chuck even though the relationship with Dan was good and she felt intellectually stimulated. This statement upsets Dan. She steals his phone to keep him from seeing, but he sees anyway. He tells her that if she cares about him, she'll meet him at the Shepherds' divorce party to tell him. She debates whether to go, but ultimately chooses to end their relationship to be with Chuck. Meanwhile Serena meets Dan at the Sheperd's and tells him that his love for "Blair" was based on the character he created but not on the real Blair, she tells Dan that the "real Blair" has always loved scheming and that it will always be a part of her nature. Dan does not trust Blair and finally decides to hook up with Serena after she confesses her love for him. Serena kisses Dan and records everything on her cellphone. Blair is reluctant whether not to pursue Chuck, but she comes to realize she has always loved Chuck and goes to Monaco to tell Chuck she's ready to be with him.
In the premiere Gone Maybe Gone, Blair and Dan see each other for the first time all summer to find Serena. Dan is angry that she left him to maybe be with Chuck, and informs her that she and Chuck have excuses but not a great love. After becoming homeless, Dan asks Blair if he can stay with her for awhile. She agrees to let him stay (Portrait of a Lady Alexander), and he decides he wants to win her back. He offers to escort her to Cotillion, but she kicks him out of her apartment and ends their struggling friendship after his sex tape with Serena airs at the ball (Monstrous Ball). Blair confronts Dan for cheating on her with Serena During Thanksgiving Dinner, Blair tries to convince Serena not to get back together with Dan because he's bad news. Serena doesn't believe her, until his serial pinning her as a golden shell comes out. Afterwards, Blair and everyone else begins to ice him out (It's Really Complicated). This continues until Dan helps Chuck defeat a newly resurrected Bart Bass. Blair thanks him for doing the right thing, but takes it back after she learns Serena may move to Los Angeles after Dan's article on her (The Revengers). In the series finale New York, I Love You XOXO, Dan is present when Blair marries Chuck, and although she is furious with him for truly having been Gossip Girl, they obviously reconcile, and his wedding to Serena takes place at Blair and Chuck's apartment in the time jump.
Dan: Look, Blair, I don't think you're that bad of a person. Maybe not my type but you're not terrible. And whatever it is that happened between you and Chuck, I'm sure you can fix it because if there's a pair meant for one another, it's you two.
Blair: You're absolutely right.. Chuck and Blair, Blair and Chuck.. who else could love me after what I've become?
Dan: Say life is giving you signs, and you're ignoring them because you're afraid of the thing they're signaling you to do. But but- then you think, what if these signs are here for a reason? And ignoring them just makes me a coward?
Blair: Signs are for the religious, the superstitious, and the lower class. I don't believe in them and neither should you.
-Last Tango, Then Paris (3x22)
Blair (to Dan): Never to be realized literary aspirations, check. Townie, you're from Brooklyn, so check. And giving up everything to become Serena van der Woodsen's stalker, check. Face it Humphrey, you are one knitted tie away from Mr. Donovan territory.
Dan: You care about fashion more than most people care about, well, um, anything. You used to send girls home crying from Constance for wearing tights as pants.
Blair: Well, somebody had to. It was for the greater good. Just like I suggest you take off that tie and shove it in your pocket.
Dan: You're an evil dictator of taste, Blair. Why deny that just because it's what your mother does?
-The Kids Are Not Alright (4x12)
Blair: I wanted to tell you... that you were right... about that thing.
Dan: And now which thing is that?
Blair: I, Blair Waldorf, need Dan Humphrey's help.
Dan: As a friend and peer, not an underling.
Blair *mumbling*: As... my friend and peer, not my underling.
-While You Weren't Sleeping (4x16)
Dan: You are the most accomplished liar I know, except maybe Jenny, but you trained her.
Blair: This is different!
Dan: Maybe, but Blair, what is a lifetime of blackmailing authority figures and casting out townies if it doesn't add up to the ability to at least feign a marginal level of happiness at an office party? You know, you banished Georgina to Belarus, I'm sure you can down a few canapés and watch the ball drop on Nate's big screen.
-The End of the Affair? (5x11)
Blair: In case you haven't noticed, I'm not exactly having fun either. This was supposed to be the best day of my life and now it's a big mess and now you don't need to be yelling at me on top of it!
Dan: In case you haven't noticed, I've been helping you the entire time and YOU'RE the one who's yelling! Alright, now I'm yelling too but you're the one who started it!
Blair: I HOPE SOMEONE YELLS AT YOU ON YOUR BIG DAY SO YOU KNOW HOW IT FEELS!
Dan: You already did! You remember the day my book came out? You blamed me for creating all your problems with Louis, AND you never even read it!
Blair: I read THE MOST IMPORTANT PART... THE PART THAT NEVER HAPPENED!
-The Backup Dan (5x14)
Dan: I love you.. you know that, right?
Blair: I do now. Let's go.
-Raiders of the Lost Art (5x22)
Dan (to Blair): So I didn't lose you to Chuck, I lost you to the idea of Chuck?
-Gone Maybe Gone (6x01)
Blair: I thought Chuck sleeping with Jenny was the most despicable thing someone could do to me but this is worse because you pretended to be better. At least I ended up with the right person.
Dan: You ended up with no one. Chuck will never be finished fighting with his father and that ring around your neck will never be on your finger.
Blair: I would rather be with no one than with you.
-Monstrous Ball (6x05)