Usually some cross lighting from behind will help out. Having black on black is hard without having them blend.
Set up a strobe to hit the BG creating a halo type lighting behind them? That would give some variation the BG to allow the subject to stand out.
Also, is it that the blacks aren't showing up because of improper exposure or just the edges of dark clothing/hair blend in to much?
I view lighting much like a painting. Put it where you need it with how much you need it and it should help.