Here's a graph from a gigantic Deadspin compilation on the subject of black NFL quarterbacks. It's of the distribution of quarterback seasons by yards per pass attempt (which is probably the single best traditional statistic for assessing passing performance) versus that season's league average. White quarterbacks are distributed in a bell curve, while there are a lot of fair to middling black quarterbacks, but a clear shortage of ones more than 5% better than the league.
This chart doesn't include running yards, which blacks are better at compiling. So, the two groups are more similar in overall effectiveness. But it does deflate the not implausible assumption that blacks ought to be better passers because they are more dangerous runners, so the defense have to look out for the QB taking off downfield.
My guess would be that the game has gotten so sophisticated that it's now hard to become a consistently well-above average NFL passer if you haven't been focused on passing and nothing much else since you were about 13 years old.
And that sort of means that you need your Dad coaching you or hiring QB tutors for you, instead of just listening to your coach. Your coach's conflict of interest is that he needs to win this weekend's game more than he needs to develop you as an NFL pocket passer. Most black guys who have played quarterback in the NFL were just about the best runners on their high school teams. So, if you want to leave the pocket, put the ball under your arm and run to victory, well that's okay with your coach even if it develops habits in you incompatible with being a Manning/Brady in the NFL. (Among other things, it's not fair to your teammates to prioritize your development over the team winning.)
I don't know that that's true, but it seems like something that could be tested with enough high school data.