Perhaps Darcy is just letting Caroline know that he will no longer be using Lizzy for comic relief.

Mr. Darcy had at first scarcely allowed her to be pretty; he had looked at her without admiration at the ball; and when they next met, he looked at her only to criticise. (Ch. 6)

After several meetings, now at the Lucas party, not only has Darcy finally decided he would like to converse with Lizzy; he has just found out that (unlike many) she would not automatically jump at the chance to dance with him. He has finally heard her sing. In short, Lizzy has finally become a real person to Darcy, not only a real person but an interesting person.