Les, the quote "and called it 'Goin' out a-niggerin'". Doesn't necessarily mean it came from the minstrel tradition. Hoover didn't invent the vacuum cleaner but nearly everyone in this country calls them hoovers. It could be that this term was originally used as an insult which was then appropriated by the objects of the insult.

I tend to support the theory of blacking up originating as a disguise rather than as anything else (although it's not uncommon for something to have more than one element in its origin). Having painted up for morris and seen plenty of other teams white people with black face paint are a lot harder to recognise than if they wear other colours such as blue, green, red or white.