Chuck suggests they speak to the "only person who knows what really happened that night," but refuses to speak her name because of how she's Bloody Mary and once you invite her into your house that Kills song will start playing and somebody will go insane or fake-kill somebody every single time. Blair gets a little scared, because she's always been jealous/afraid of Georgina. As close as she and Serena are, and as unbreakable as the Core Four always are, Georgina has a piece of Serena that nobody else has.
There's a thing in Serena that not even Chuck can handle looking at, and Georgina knows it intimately, because she embodies it. She's the chaos on the other side of Serena. Serena kept that part of herself away from Blair even before Pete Fairman died, because that thing is what would destroy Blair if she ever looked right at it. Abandon -- the gift Serena gives her -- is only nihilism from another angle, and you could say that now Georgina Sparks is a living signifier of the thing that Yale, and Nate, the momentary flirtations with the Colony Club and the van der Bilts, have all been about avoiding. What controlling her food used to be, before Cyrus Rose took that away, and told her she was enough, and loved her enough to make her believe it. That thing that used to be Chuck, until he fought a dragon of his own and became too real to attract Blair without scaring the shit out of her, and became one of the poles that keeps her in alignment, for the time being.
Because every time you think Blair's about to give in and just have her crisis, she's been secretly setting things up to be even more shallow and unemotional, or find another way around the obstacle, and it's always a surprise. So what happens when she really does let go? Because it's not about Serena, this time. Serena fought that dragon, and won, and her prize was that she didn't have to be with Dan anymore. Her prize was the gift she gave Blair, at the end of last season: not falling, but dancing. Georgina let Serena's spirit go, because she passed the test; Blair has no way of knowing that this time, Georgina's coming after her. The song is going to play for her; her bubbling craziness has been working since New Year's Eve to get Georgina back here, pulling sparks along a secret thread. She's earned her fever.
This is my favorite non-Georgina scene in the whole thing: Dan lies on the living room floor with Vanessa, totally blitzed on three beers, feeling weirded out about how he's suddenly not going to Yale again. Vanessa fake-apologizes for "letting it out like that," but covers her ass by pointing out that at least poor Rufus won't have to tell Dan himself. They lie on the rug in playing-card alignment, arms at opposing angles, beautifully composed; Dan's necktie lies artfully askew on another parallel, as the camera pushes down on them and Xu Xu Fang (last heard playing more than one song, note, in accompaniment to Chuck's post-funeral meltdown about winning his father's posthumous approval) melts everything into a soft drunken sinister dream.