It's in their genetics, or at least - far more likely to be in their genetics than with other races. Black athletes, as well as any other race, generally have fast-twitch muscles fibres as this is immeasurably beneficial. And it's not "stereotypical" or "racist", it's a fact that black people are, on average, better at running and white people are, on average, better at swimming.
ps: white people are, on average, better at swimming because generally they have less dense bones and can move quicker in the water!
edit:
By the way, descendants of slaves are kinda genetically more advanced than what they would have been. To survive being a slave meant you were extremely tough and it was years of "natural selection" sped-up over a lesser time span. If you were weak, you probably would have died very soon after being caught...
^ That's just to add to some of the similar comments about the slave trade thing on here.