Another thought: could you have a black face as a member of a group of Klu Klux Klansmen? It's an extreme example but it shows that the colour-blind theory does not work without fail.
Nah, I still don't see it. Maybe some casting is directors trying to be right on, I don't know, but I'm happy to accept that they're picking the best out of whoever auditioned. To question it on the basis of it being in some way anachronous doesn't hold water for me, if that actor is giving a good enough performance, that's all that matters to me. In answer to your question about casting a KKK member, I'd say: I don't know. I share your instinctive feeling that it wouldn't work, but I'd have to see it. It'd be the supreme test of that actor's ability but I wouldn't rule it out.
It simply boils down to whether the actor does a good enough job. It's a shame if people let other factors get in the way of that.