"If guising was done predating minstrel shows, and the materials likely used were coal dust and/or burnt cork, how effective would those materials have been at covering the face? It seems to me, if it happened and those materials were used, the end result would have been streaking and smudging and not a full face cover. In which case, today's practice does not mimic a supposed or reported tradtional pre minstrel show history."

Agreed. I raised a similar point earlier. Current stage paint, is not akin to a supposed 'sooty' face. If we feel the need for authenticity, then we aught to be following the principle of 'using what is to hand'. If we buy the guising theory of using readily available local materials to diguise the face - then the colour of whatever is used, is purely incidental to those found materials.

As Pip's repeatedly stated, the costume side of things is very different now, than a few mere decades ago (when likewise very few sides even used face paint). People have adapted their costumes to include items easily available to them, so no reason to not do same with other facial elements of the disguise - IMO.