Blair comes home to Chuck with shoulder pads overwhelming her delicate frame, and Chuck seems surprised that she wrapped up her foray into prostitution so quickly. Her face would seem to suggest that, or something worse, has happened, so he sets up all his little domino lies and self-justifications and just starts firing them off robotically without even considering if they fit the context of the conversation that's actually occurring. B straight up asks if he really sold her out, and Chuck says she is the betrayer because she's the one coming home from the Empire.
Which, her intention was to go through with it, so he's not wrong about that tiny fact, but it's really immaterial. Blair reminds him that, just like with the boy-kissing, she would have done anything if he just asked her to, including I guess Jack, which would be weird if we were talking about any other human beings but here seems probable. Chuck tries to explain his mental alibi about this, which is that if she were in on the plan, then they'd all be in on the plan and Jack wouldn't feel like a winner, and the deal wouldn't go through. Blair is disgusted, because that is disgusting, but also because she likes to be in on the scheme and not a pawn. If your whole life is chess, then there's nothing worse than getting played: And in that case, this is actually worse than the prostitution part.
"I can't let my feelings cost me all that I've built," Chuck whines, and Blair points out that what it's costing is her. "All I ever did was love you," she says, heartbreakingly, and then Chuck plays his next card, which is her original bluff-call that got them together in the first place: "The worst thing I ever did, the darkest thought I ever had." He says he is redeeming that coupon, as it's self-evident that this is the worst thing he will ever do. Blair is very sad and very beautiful and very much has already left the building, watching his face as he says that, and you can see her swallow it: "I never thought that the worst thing you would ever do would be to me."
Which is where you apologize, but he's so cornered and has spent so much of the day justifying his shit, that he pops out a "You went up there on your own..." before he even knows what he's saying, and she slaps the shit out of him and bounces, because just because you've said something in your head a hundred times doesn't mean the person's going to hear all that mental effort on your part when you say it out loud. Also, eye-for-eye is not what's called for here. And most of all, calling your girlfriend a literal whore is not ever going to be an effective strategy.