Why do black people keep saying "we built this country"

This is something I've heard black people say before, but I've never heard it from people of other races. I get that they were brutally abused in the past but why do they just blame "white people" when I'm pretty sure it was only white people of a certain class that owned slaves. I don't condone slavery, I just don't think it's right to put collective blame on an entire race of people.