This is not true in the least. Anyone who knows anything about Hollywood and the way it works will tell you that. Hell, anyone who's ever read any of those leaked Sony exec e-mails will tell you that.
I don't see how that somehow translates to being a bad thing about more black people getting roles. The fact that we are beginning to see blacks a little more than before has nothing to do with any other group's lack of representation. More to the point, why do blacks have to get less representation in order for Asians, Hispanics, etc. to get more? Why not chip away from some more of the Caucasian representation (who, by the way, are ridiculously over-represented in the mainstream media)? I think you are thinking about this in the wrong way.
Also, conspicuously absent in your discussion is the status of black women in Hollywood. It's very telling to me that whenever this conversation comes up, no one ever even thinks to mention black women (specifically dark-skinned black women) and how Hollywood often goes out of its way to ignore them. And even when we do see a representation of a black woman, she often plays on people's stereotypical ideas of black femininity. That they are angry. That they are ****es and homewreckers. That they are welfare queens, etc. I suppose that is a reflection of their status as a virtual non-entity in Hollywood.
My two cents.