Which is another answer, entirely. The opposite answer. The one she kind of wanted when she walked in. I cannot say enough good things about the acting in this scene, but the writing's also there. Like Blair, just unbelievably so: "This fairy tale is... Complicated."
Ah, now I get the thing with the dog: She saw him handing the dog off and assumed, as we were probably meant to assume, that he was getting rid of it. Meaning that he is still in no position, because he's too busy snorting and fucking to take care of a dog, much less a broken fairytale. And what's awesome about that is, she wasn't actually ever avoiding the question: She was telling two stories at the same time, and freaking out about both of them, and loving both of them, and she didn't want either of them to end. I wasn't entirely sure which angle she was playing until the puppy, who comes running in right on cue. Her eyes light up, a little. The dog's name, of course, is Monkey.
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.