Chuck admits he was just getting Monkey fixed, and that's the upside: Maybe she's doing this wrong -- maybe he's healed enough to care for someone else. But it's Chuck: "I thought it was the responsible thing to do." And that's the downside. There's another answer for you: The nastiness behind it. The planned, conscious clarity that would snap out like a viper and say something like that. So Blair nods, because she is clearly doing the right thing: "I should go," she says: That was uncalled for, her face says. He doesn't disagree with either statement.
She stops at the door, just to give him something. Some tiny thing that might help, if this is the person he's turning into: "There is a part of me that really wanted it to be yours." It's not what she means, but it's the best way to say it. She surprises him with kindness; he's never prepared for it. That's how she gets him, every time.
I mean, just beautifully acted. And not only that, but it's the first time she hasn't been juggling and throwing jazz hands and acting crazy, so you get to retroactively see exactly how much personal internal work she's been doing this whole time, if you didn't pick up on it before, and how she's well aware of exactly what's going on. You see the choices on her face. And it occurs to me that this three-way triangle is even more subtle than it seemed: You have Chuck, which Can't Happen, and Louis, which is only about Blair, and then Dan which is secretly happening, and she's got all those balls in the air, plus the baby, and I don't know. Sometimes you can forget everything that's going on underneath because she does such a good job of tricking you into thinking she's shut down.
And then too, if you take off the shipper glasses for a second, there's a symmetry here I wasn't really conscious of the first time, which is -- in my memory at least -- how that awful incident was composed left-to-right, with Blair nearer the door and he throws her on the couch and it's so terribly gross and claustrophobic that you get scared. I could go and check, but I'm fairly certain that conversation started with similar staring, and similar "We need to talk" stuff going on. And he lost it, and it was shameful for everybody.
And now we're in a perpendicular space, with him on the left in the background and we're mostly standing with Blair near the door again, but the house is like this mouth swallowing him up (the camera even backs away from him once she's gone). So visually you have them in this sweaty awful tiger cage, which is now a lonely open space, right-to-left, and she's handing him the next step in getting rid of him -- the Window Incident was also about her telling him, about the engagement that time, before anybody else could -- and I don't know.