Well, Blair/Dan was kind of getting a teasing, slow burn build-up all season, and I this summer I rewatched the whole series in like 2 weeks, and with season 4 canon...you can kind of follow that they've been building up all along. Anyway, in the season finale this year, Vanessa found this manuscript of Dan's called Inside which I guess is supposed to be a thinly veiled fictional commentary on the UES (with characters like 'Sabrina', 'Claire', 'Charlie Trout', etc.), and dared him to get it published and he told her to put it away, and not talk about it to anyone, but she took it to a publisher anyway (and then moved to Spain). She asked him why he was hiding it, and said, "I think *someone* might *like* the way you portray her, and I'm not talking about Serena..." And then she asked him how long he'd been working on it, and he said off and on, for about five years (i.e. longer than the canon of the show).