Black players dominate all other positions on the field...why is quarterback different?
Equal parts racism kept them from the QB position because they weren't deemed smart enough to master the offense, and equal parts athleticism because they were typically skilled enough to play other positions better than their peers right off the bat.
As the game has evolved, and racist notions have slipped by the wayside, there has been an acceptance of putting athletic players at the QB position. This is especially true at the HS level, where the most talented player physically is often the QB. They can have the largest impact on the game from that position. Because these gifted players are put in as QB because they are head and shoulders above the competition, you are seeing more and more guys listed as "ATH" when being recruited by colleges.
The idea that black players are too stupid to play QB is still an issue, but MUCH less prevalent than it was even a decade ago. The way things are headed, it won't be an issue at all within 15 years as black players will dominate the QB ranks as they have most everywhere else on the football field. The pocket passer is a sitting duck and an invitation to be injured in modern "basketball on grass" football.