Darcy did not say the Meryton assembly attendees were uninteresting until the next day. We do not know why he refused to be introduced to anyone at the ball, but he did walk around the room - perhaps his habit of listening to the conversations of others is what gave him the idea that Netherfield's neighbors were uninteresting.

(Although, if I were saying anything interesting, I am not sure I could keep it up when noticing a haughty stranger within hearing distance. Anything that can make someone think or laugh, might also make them angry or condemnatory. I would probably switch to a bland topic until he had passed!)