The facts are black Qb's, in general have under performed.
I think that this is far from a fact. A black quarterback has been MVP of the league. A black quarterback has won the Super Bowl. Several black quarterbacks have played in the Super Bowl.
You're certainly right that there have been more successful white quarterbacks than black ones, but to determine whether or not black quarterbacks have underperformed would not involve simply saying "fewer black quarterbacks have won super bowls than white quarterbacks." Instead, the argument would have to be that "the
percentage of black quarterbacks who have won super bowls out of the total numbers of black quarterbacks in the NFL is smaller than the
percentage of white quarterbacks who have won super bowls out of the total number of white quarterbacks in the NFL." The same sort of statement would need to be made for other methods of evaluation, such as pro bowl appearances, super bowl appearances, MVP awards, etc. Would such a statement hold true? I have no idea, and neither do you (unless you're about to break out some as-of-yet unpresented statistics).
Edit: Since Doug Williams won a Super Bowl in 1988, 12 white quarterbacks have won. Has the overall ratio of white:black quarterbacks in the NFL been more or less than 12:1 during that time? You don't know. This is the sort of information that would move your argument out of the "racism and bigotry" category and into the "evidence" category.