As black as coal

I have often heard the phrase "As black as coal" referring to coal as it comes from a mine. But it has suddenly dawned on me that I/We may have been misconstruing the phrase and it really is "as black as Kohl" meaning the ancient black eye make-up made from soot.

Not possibly. Coal has been a stock comparison for blackness for at least four hundred years - Shakespeare uses "coal-black" a number of times, and it certainly wasn't a new or original comparison then. But the first known usage of "kohl" in English is dated by the Oxford English Dictionary in 1799, centuries later - and I very much doubt if it was actually known and used by any English-speakers one other than a few Arabic scholars and other such specialists till half a century ago at most. (VSD)