The 'disguise' theory is only one of several, alongside the 15th century influence of Moorish dancers and the 19th century influence of minstrel shows (this last theory also relates to the Bacup dancers). Certainly for a modern audience, blacking up is synonymous with minstrelry, so its no surprise racist connotations are assumed. I'd suggest that regardless of the theories, if you black up at least expect some shit to fly.

I doubt tribal customs whereby black people paint their faces black are going to attract criticisms about racism, Topsie, unless you're suggesting these Papuans are being racist towards themselves! Though Robert Hornback compares border morris to crude parodies of African tribal war dances, predating minstrelsy and contributing to that tradition.

There's nothing to stop modern morris dancers in a multicultural society painting their faces in other colours, as some do. It's not like BM hasn't been reinvented in other ways...