What happen is that almost none of those with European ancestry take sports as a serious career choice, and of those that do, they may demand a higher price or be more difficult to deal with (because they have other choices available to them) than black athletes of similar ability, which would also create a selection bias for blacks.
Black dominance in US sports is mainly a matter of selection bias, both by the individuals and sport institutions.
Really, I doubt it. There are lots of high school track teams, and lots of people who run on them, but invariably the people making it to the Olympics are black. And, it's not just the USA. Countries like the UK and France, that have even smaller black populations, send black sprinters to the Olympics too. Is selection bias working in every country as it does in the USA, despite very different cultures.