Hi Sven, a style color of full black (rgb 0,0,0) will be black for wires like curves and edges but map to white for shaded surfaces.

See here for some info on why:
http://moi3d.com/forum/index.php?webtag=MOI&msg=11202.7

If you do want to make a black surface use something just a tiny bit above 0,0,0 like 1,1,1 .

- Michael