Blair enters her piano parlor to find Chuck playing beautifully, and nearly faints from the awesomeness. She's wearing a dress that seems to have found itself halfway between Blair and Serena, and they both whistled to it, and it ended up reluctantly coming to B, and she reluctantly put it on. That's the scenario I'm envisioning to explain B in a super-short satiny S dress like this. Chuck says that he has decided he wants to choose her Snowflake date, to prove that he knows her better than anyone else. OMG these people are like the Inuit with these million words for I♥U, if the Inuit believed that actually saying the word "snow" would cause the entire universe to end.
B agrees, on the condition that she be allowed to choose his date, and that if some vaguely defined and suspect rubric is met, one or the other of them will prove to have been "paying attention to the other's desires," and thus will win the bet. These people just... stopped even trying to make sense at some point. I mean, it's very realistic and high schooly and all that, but the fact that this even makes a little sense to either of them just proves how overextended they are emotionally at this point. It's like the duck pond thing with Dorota, only the opposite. "If you even for one second admit that you are not having fun with somebody else, I will freak out." Blair names her stakes if Chuck likes his date -- his nouveau riche limo, for a month -- and Chuck is awesome: "Fine. And if you like yours, I get Dorota." Yeah, they totally just bet a human being. As with any of their other games, I mean, it's not a new concept with them. Chuck caresses B's face and takes off; Dorota realizes she just got sold out, and gets goosebumps of horror.