Although you say that 'blacking up' is not an effective form of disguise, I would challenge you to recognise a close friend if they were to appear at your door one night, blacked up and wearing a costume you'd never seen. It's incredible the difference that a blackened face and limited light will make to facial features, hair or eye colour. An example is my old friend Bob. I would have sworn Bob had ash blonde/dark grey hair. The day I saw him 'blacked up', it was pure white... he looked like a badly poured pint of Guiness, and I only realised it was him when he put his glasses on.

Disguise or 'guise' is indeed related to 'guising' or 'guysing', the act of going around in disguise - and is the origin of the word 'guiser' or as we spell it now, geezer.

LTS