How light or dark your diffuse is in the end will depend on how intense your specular is. It's probably a good idea to work on both at the same time as a ratio so that you never exceed the total amount of light that is hitting the surface (so if you take this material and want to say that this is what it looks like as a…
As a general rule the diffuse should be the color of the material when it is lit, which will never be black (no material is perfectly black when lit, the closest that has been done was in a lab, with carbon nanotubes I believe). The 'black' color we see is always relative and dependent on the lighting of the environment as…
If it's medieval you could go the burning torch route. With black metal I've found that putting most of the detail into the spec map instead of the diffuse (and using a quite plain diffuse) works well.
Here is an example of what I have. I figured darkening up some brightly colored metal might help but I think I'm way off? BTW This is diffuse only, not that I do not apperciate the advice on the other formats like Specular maps n such but if I don't get the Diffuse map correct than I cannot even think of making those yet…
While I'm here actually, if you looks at the tutorial that @TicoTaco posted his finished result is this: Notice how the diffuse is pretty much just dark grey with the odd bit of slight noise? Pretty much all of the detail is derived from noisy Normal and Specular maps.
I never make materials in Max/Maya beyond Diffuse and Alpha maps. As far as I'm concerned, I just need enough information to UV unwrap it properly and then do all of the fancy shading in UDK - changing textures in Photoshop and reimporting them as I go along.