In the late forties, early fifties,one saw many chimney sweeps. We didn't have central heating, everybody had a coal/wood fire, and the chimneys needed to be swept regularly. Sweeps were either on a bicycle or on foot, with their special brushes over their shoulder. Also, men delivered the coal in open sacks hoisted onto their backs from lorries, and 'shot' the coal into one's bunker behind the house. They wore thick leather protective headgear that hung down their backs. Both these type of workmen had completely blackened faces. Not just smudged, but totally black. And in mining areas (my aunt lived in County Durham for example) the men came off shift covered in coal dust, again, black as the ace of spades. No showers at the pithead in those days, but a tin bath in front of the fire in the living room. It did give the appearance of something a bit sinister and strange, all these unidentifiable black-faced men. So blacking-up for Morris dancing would have been seen as an extension of this, and nothing whatsoever to do with African-origin people at all. In fact, while living at the edge of London, I had never seen a black person, until Jamaicans began to arrive. I used to think as a child that they were chimney sweeps or coalmen, until my mother put me right.
My point (after this long ramble) is that men with blackened faces were around long before we saw any African-origin people. So nobody was parodying or imitating them in any way.