completely depends on the circumstances. If it is a kid or a close friend, you can offer to take that move back. In a tournament or against a stranger, you don't do that, of course. Even though you think it is a blunder you act as if it is a deeply tactical thought through move, contemplate on your possibilities and then take the piece. Who knows, maybe it is actually a trap?
Always keep your pokerface. That's polite and professional.
I usually think about a minute or two to make sure that there is no traps then i capture the blundered piece.
on that note, I dislike winning because of a blunder. I feel that a blunder can destroy an exciting game. So I rather have it taken back and forgotten about.
The blunder in the game I played was pretty obvious. The guy played 29...Qg5?? and lost the rook to 30.f4.
Is it rude to smile when your opponent makes a blunder in an over-the-board game? I tried to look indifferent when my adversary dropped a rook in an otherwise equal ending, but deep inside, I felt like Bluto in 3:10-3:11 in the clip below: