To me, it's all a question of the impact actually achieved by the particular play/performance.

If an actor can carry a role off in the context of a certain production of a certain play despite (or because of) blacking/whiting up, more power to him.   

If Bert Lahr was able to be believable as the Cowardly Lion in the magical context of The Wizard of Oz movie, that speaks not only for his expertise as an comic actor but for the costumers, makeup people, cinematographers, director, and all concerned--not to mention the original author. It also speaks for the readiness of the audience to accept fantasy.

But in a play or movie meant to show the drama of fear by realistic people threatened by a lion in the jungle, poor Bert is not going to get there, despite his talents. You'll have to have an animal actor.

A whited-up actor playing Hamlet is perfectly legitimate if he can, in the context of that production, make the audience feel the drama, feel the personality of Hamlet, and for that time forget the actor's race. Likewise, a blacked-up white actor-singer playing Porgy is permissible if he can carry it off.

The whited-up black actor might possibly achieve the necessary illusion in a cloak-and-tights production of Hamlet but fail in a modern-dress production (or vice versa). In the end, if he fails, he fails.

There are only two rules:
1. The theater is the realm of illusion.
2. Nothing succeeds like success.

Dave Oesterreich