One of the best ways I've found to work on dark and black skin is to first use zenithal highlighting then follow it up with two or so layers of a really thinned out black (a nuln oil wash may work as well, but I haven't tried that)
The zenithal will help with natural highlighting while making sure the lighter areas don't turn into a flat black. If you want a speedy result, follow all that up with a drybrush highlight of mechanicus standard grey over the whole model. To really make it pop, pick out the really light areas with a light grey (administratum grey) and then an off white (pallid wych flesh).
Here's a reddit thread I found of the same exact question.https://www.reddit.com/r/Warhammer40k/comments/2z8sl1/guide_on_painting_black_bloodthirsterdemons/