>if it's true that blacks are still being discriminated against, and nobody is allowed to compensate for that by taking action, what else can we need to do to solve the pre-existing discrimination, assuming that it's cultural and difficult?

You won't get many responses to this question. Stated plainly: If we aren't allowed to engage in some form of positive discrimination, we aren't allowed to compensate at all. To some people, that's how it should be. I don't agree, but I understand why they may feel that way. We all just want the best for ourselves and our respective groups, I guess.