Only about 12% of Americans is black, so 8.5% is an underrepresentation, yes, but as you note, there are social and cultural factors explaining why other sports are generally speaking more popular among black people than baseball. Taking that into account, seems perfectly alright to me, lacking evidence of anti-black discrimination (which I'm not saying doesn't exist, but you don't mention it at least and I don't know much about baseball myself).

The thing with race in the US - in some other countries it's the same way, but certainly not in all - is that it's mixed up so much with social and cultural factors. Obama is a case in point; calling him an "African-American" is obviously correct literally (half correct, anyway), but if "African-American" refers to the socio-cultural group(s) rather than to pure race, he's only a member of that group to the extent that he assimilated into it when he was already an adult. He's the first black president - only partially an African-American president in the usual meaning of that word, though.

Which is why, in the US, more than elsewhere (except for South-Africa, I suppose), race disparities in sports have to do with sociocultural factors more than with any more strictly racial factors like genetic advantages or racism in the sport. Which doesn't mean those factors don't exist, just that they aren't the primary factor.