An important comment upon the use of black undercoating: many people I have spoken to generally agree that while it is theoretically sound, in practice colours such as red or yellow on black will look dull and dead. In general, you should use a black undercoat only if a significant part of the model, say 75%, is in fact going to be black.

If you are in fact painting a mostly-black model, then it might also be a good idea to consider that you can give the appearance of shades of black by using very dark blue.
The new Eldar Dark Reaper models are good examples of where this can be used to good effect.