Dont use pure black "out of the Box". Mix it, using blue, red and green with a dip of brown. This black can be mixed with light blue for highlightening or with black for deepening.
Well a more realistic black skin tone would include brown instead of grey (or a mix of both). Then take black and keep mixing in more brown and build up your highlights like that.
Practise makes perfect (well almost anyway... ~_^)
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