Blair, now torn between the real and present pain that Chuck's going through and the very clear mental place Serena is charging into, just stares. Chuck heads down to the bar for a drink, telling them they're welcome to the hotel safe combo if they want to violate him a little bit more. When he leaves, Nate's like, "...He'll be fine," and Serena's like, "[I'm] acting CRAZY!" Blair mans the eff up, just incandescent throughout the episode, and tells them in no uncertain terms that they will leave him the hell alone. "You didn't see his face when he confronted this woman and she sent him away. He's had enough pain in his life. If he doesn't want to hear her story, then I'm going to respect that. And so will you."
And I mean, there's a whole motif here, the "hear your story" thing, that comes up in every storyline -- even Jenny has Damien's daddy issues to contend with -- and it's a fairly sensible way to collect the threads. Because Chuck is so next-level that he's making this sort of existential choice to just block Elizabeth altogether: His mom or not his mom, it's the story he can't handle. And Serena's generating a story a minute, and has been doing so since Santorini, each one sadder and more pathetic than the last.
But you also have the two couples substituting sex at various points for the Story -- Serena avoiding telling Nate her basic thing, Blair and Chuck fucking instead of talking later on -- and Dan/Vanessa afraid of each other entirely because of the untouchable Story of Them, Rufus and Lily telling and avoiding telling stories... For a show about surveillance, this jump to the primacy of personal narrative is really exciting. Chuck doesn't want Elizabeth's story touching his own, and Blair's bravely telling a whole new story: The one where she turns off her innate suspicion and natural curiosity, her puzzle-solving MO, in order to say that if Chuck says so, the past stays the past. He's allowed control over his narrative in this case, because of how dark she knows he might go.
All of which is something that, until now, she would have been unable to comprehend, because her single life strategy is inventing new stories to solve the problems of the old stories -- putting herself in movies to escape her own violent mommy issues, for e.g. -- and maybe more than anybody on this show (witness Bad Serena constantly reappearing, even though the basis of this entire show was her New Serena story) has actually managed to find grace a few times by doing so. But if Chuck says, "I don't want a Cyrus Rose-type replacement, or a Lily Bass, I'd rather have no mother at all," she has to respect that, because she's all-in. And it's one of the strongest things she's ever done, and I love her for it: At least we'll burn trying.