I think it's a relic of primitive culture where one masked one's face and 'became' something else, maybe a spirit or representation of a god.
Having watched the Witchmen and other Border Morris blacked-up sides, (and Red Leicester), it's strange and rather wonderful how the 'magic' is generated when their faces are disguised. When my husband first watched the Witchmen, he was astonished and remarked how similar it all was to many W African costumes and dances. He couldn't believe we Europeans would indulge in such a 'primitive' exhibition. Masking is very common I believe throughout many cultures. Funnily enough, I've watched dancing in Senegal where the (very black) Wolofs and Serrers 'white-up' and become various spirits. And I most certainly did NOT object on racist grounds that they were insulting white people!