The game is just too fast for sluggish white guys anymore. Where would a white QB be right now behind that Seattle line?
The big thing is extending a play, something black QB's can do because they are athletes, most could play another position, Tom Brady/Peyton Manning/Joe Montana/Dan Marino etc etc were "just" QB's only.
The defense knows where those 4 must be, so no need to worry about outside containment, while a RW puts more stress on a defense because of that...ZOOM!
Look at the NCAA right now, the top team in the nation, a black QB. The top ranked team in the NFC, a black QB. Defending NCAA champion, a black QB. What QB has played in more SB's lately, a black QB.
Sure, Tom Brady, Aaron Rodgers (who is mobile) can make it work but that breed is dying out.