I find myself in partial agreement with both Robbin and Jeremy.

I think it would take something very specific and directed to undo the impression caused by Darcy's initial insult. While the exchange between Darcy and Elizabeth may be seen as flirtatious, it also borders on the argumentative from Lizzy's perspective, so I do not find it at all surprising that she does not see this as flirting. His offer of dancing a reel, in this context, was also not seen as serious.

Darcy's interest is obvious to Caroline (and probably the others) in part because he was open and explicit about his admiration of her "fine eyes," and his actions would be understood in that context. Lizzy was not privy to that information.

IMO the point at which Lizzy should have understood his change is the ball at Netherfield, when he asks her to dance. They do dance, and in such a public place even Lizzy could not presume he was asking her in order to despise her taste. We have no evidence that Darcy danced with anyone else (except, presumably, Bingley's sisters), so in dancing with her he has singled her out.