I'm going to ask my bf outright if his family is racist
What makes you think they are racist?
I found several pictures of them dressing up like white people (slavery era style)
You should make sure before you do something like that. It could be a misunderstanding and they might not be racist and you shouldn't treat your friends family that way.
I don't care if I hurt him, I'm gonna ask him anyways.
Well just be careful that you don't insult him and his family, he might be mad that you even asked.