Black history museums are interesting because they offer an insight into the achievements of a transplant group in a country where they are a minority. It's not really about their history prior to immigrating. It's about their history after they've gotten into this new country, and the story of their integration.
I don't think it has anything to do with race, it's a cultural thing - groups that identified with a common history in a strange land.
For example, in the UK, despite being also predominantly white, I'm sure there would be a remit for a polish history museum, or even an irish history musuem.