This was discussed in several other threads and what I gathered from that is that many feel that while Darcy was rude in making his comment, he wasn't being cruel or needed to make an apology to her because he didn't realize his comment was overheard. However upon reading this chapter again, it occurred to me that he makes eye contact with her and looks away, thus he'd know that she was nearby and was (at least possibly) within hearing distance and presumably he's clever enough to know that she may know she's being spoken of (even if she can't hear the words) by him turning round to her and looking at her and looking away. Presumably he didn't shout it, and I daresay Lizzy wasn't leaning over to eavesdrop, so it was just his utter lack of interest or concern for anyone present not of his party. I am now persuaded that Darcy was a bit more of a jerk and less "shy and/or reserved" than I had previously considered. Oh dear!
(Emphasis mine)
"Which do you mean?" and turning round, he looked for a moment at Elizabeth, till catching her eye, he withdrew his own and coldly said, "She is tolerable; but not handsome enough to tempt me; and I am in no humour at present to give consequence to young ladies who are slighted by other men. You had better return to your partner and enjoy her smiles, for you are wasting your time with me."