Most of the black students are probably African immigrants. I see it all the time. If you just browse through LinkedIn and look at black Harvard/Ivy League alumni you'll immediately notice all the African immigrants
This is very true, and speaking as a non-black/african person, I noticed they're very different culturally. They carry a sense of national pride related to their home country, and draw from it constantly. I'm thinking about Nigerian immigrants specifically.
Yes, that's a very interesting phenomenon that should be taken into account. Likewise I've wondered what the sub-demographics are of Asian ethicity students. It's a little ridiculous to lump children of Vietnamese boat people in with nouveau riche direct from China.