My high school art teacher once told me never to use flat black as it looks extremely unrealistic. Instead mix a really dark blue and use that -- with red, yellow, and blue of course.
The black paint you are using is probably a synthetic or fabricated pigment that is extremely black. If you go to the store and check out any "black" clothing, even black ninja suits, you'll find they really aren't quite black. They are dark grey, very dark grey.
In the days of mordheim there probably wasn't black dye. If there was, it was probably just dark blue. I would venture to say that an extremely dark blue would look real nice on your models. You could probably convince me without any effort that your warband somehow managed to create or get ahold of the deepest darkest black dye (After all, they have a posessed with a tentacle for an arm -- anything is possible). That won't change the fact that flat black looks quite bland.
Get a second opinion as i've never actually painted a flat black section on a model, so I may be going in the wrong direction. It may be just as easy as highlighting with a grey/dark-grey color.