Knights "Popular Antiquities" (available as a free download from Google, and well worth reading, especially if you should be fluent in french and latin! - but mostly in english) quotes from "Round the Coal Fire, or Christmas Entertainments" (1734)

Then comes Mumming, or Masquerading, when
the squire's wardrobe is ransacked for dresses
of all kinds. Corks are burnt to black the
faces of the fair, or make-deputy mustacios,
and every one in the family except the squire
himself must be transformed.

However in reference to May Day (in London), Knight only refers to young sweeps dressing as girls and using brick dust as make-up (or as he says "paint").

More regarding "blacking up" I have not found, but plenty about disguising for mumming, and it could well be a poor man's way of masking.