I think 'blacking up' or 'whiting up' is basically a means to disguise the actors or dancers. In Scotland at Hallowe'en children come round with blackened faces to sing for sweets, and they're called the Guisers. It obscures the individual personality and renders one anonymous and mysterious. Far from being objectionable or faintly racist, I've always found Morris dancers of all types exceptionally friendly and welcoming to my black husband. If he doesn't mind their black faces, why should anyone else? Who knows, he might even one day join a side!