Using dark grey to lighter ones can work.
I prefer to use other colours to make the black pop more. It still looks black, but has a little something extra.
I like to use dark blues or dark purples.
Black base, 1:1 black + dark colour, just dark colour, then add a little white or something similar for extreme highlights in areas of interest. Face, hands etc... It's simple. Or you can just take what you have done and do a dark glaze or wash. Maybe thin a wash with lehmaia (or however you spell it) medium. Use some black wash mixed with a little blue or purple. Thinning a wash with water tends to ruin the wash as they are not ment to be mixed with water. The medium keeps the consistency of the wash, but just thins it a little so that it isn't so strong.
There are two things that I love. Kicking ass and chewing bubble gum... And I'm allllll out of bubble gum!