I think this is on the mark. The roles were not written as specifically black, but when casting took place, black actors were cast, because the casting director was being colourblind. This sounds very noble, and in a way it is. The problem is, if you act entriely colourblind, you are blind to the way your own prejudices form your opinions, and as a result, end up consistently casting blacks as victims. Which can make you part of the problem, when you were trying to be part of the solution.