Seeing as you asked, there might be some racial advantage in that particular event but there is probably quite a bit more in terms of cultural effects going on - you can see the same "selection" (albeit in reverse) in terms of swimming or cycling, where there have been astonishingly few black medallists.
If a black child shows some athletic ability, the chances are they are going to be directed down ways in which society "knows" they are more likely to succeed and where society feels comfortable in supporting them - so football, track and field athletics (though only certain events), boxing over here and in the US basketball and american football too. Obviously once they are along those pathways they are expected to conform to the "ideal" ie: power, strength, pace, physicality etc etc, and if they don't live up to that stereotype (as opposed to identifiable performance) they are probably less likely to make it. If they are talented at something like swimming, cycling, hockey or even tennis then it is probably going to have to come down to a really determined parent to get them to the point where they can compete.
The best example of this I can think of (and apologies for repeating myself) is Rom - how many managers looked at him and thought "Drogba" even though you only had to watch him for ten minutes to see that holding the ball up, bullying opponents and so on was not his game. If he played like anyone at all, I thought he was much more like Henry (chasing balls played into space, preferring to go down one side of the pitch rather than down the middle etc). How much has his career suffered because of this?
To return to the 100m, this is probably the same effect but in reverse. If you are a white lad who looks like he has talent and might grow into a good sprinter, are you going to be directed down that route or are they going to think you are more likely to get success in the long jump / hurdles / football? I think we all know the answer to that one.