I think one of the problems is that the characters aren't written to be African-American, they are just that after they got through casting (and even there they weren't looking for African-Americans)... There is no hint in the sides that either Gordon or Jake or Voctor were written to be Afroamericans (and during casting they were chosen because of their acting skills...). There is no real character which was meant to be Afroamerican.

The real problem they do have is women, no doubt about that.
But I doubt that letters to Kripke will help as long as the network exces want it that way.
I think they are trying with Ruby, problem is she is a demon and not an actual woman anymore.