There's a picture in the 4th ed. chaos codex that I've always thought would look great for a black legion vehicle (sorry for giant, terrible picture):
You've gotta do something with those big, flat panels if you're painting it black; a big chaosy symbol would look great. Go primarily yellow over black, and you can pick out little areas of gold on the gun barrels and the like.
Failing that, you could weather it to hell. Scale-model style.