Hi,
I think this ties in with the other thread of creating your own
dark colours black for want of a better word.
I would suggest as with any dark area, to work from a mid tone.
In the case of very dark hair, may be a blue/black leaning towards the blue
then using pure black only for the darkest shadows i/e behind ears,
the base of the skull very sparingly. Higlights, more blue and if needed
blue/tiny amount of white.
Like every thing else experimentation is the key,try a few things
out on scrap material until you are happy enough to put the result
on your figure.
What figure are you working on?
Frank