In the NFL, the quarterback position is usually played by a white guy, while almost all the running backs are black. My guess at explaining this involves subtle racism at early levels. When a highschool coach sees a good young black athlete, he thinks "running back", while a comparably good white kid is more likely to be put in the more mind-intensive quarterback position. Perhaps the kids themselves have different aspirations for which position to play, though I didn't see any evidence for that on the playground in high school.
It seems to me that there's a less-explicable difference among linemen. I have no real data here, I think I've seen way more blacks on the defensive line and more whites on the offensive line.