"It is the disguise that is the essence of the tradition, not the colour."

Agreed. Plus the *style* of modern blacked-up faces doesn't much resemble a traditional "sooty" face anyway. Though, it does rather like the kind of stage make-up worn by B&W Minstrels.

Midnight blue or ivy green, would serve the same function IMO, without leading to confusion.

Having said that the modern "witch men" look quite gothic in all black - so the blackened face in that instance works well with the rest of the gothic/pagan costume.