Both the Raven Guard and the Ravenwing are known for wearing jet black armour, but in practice you don't necessarily have to do this.
Painting with pure black is difficult because it leaves you no posibility for shadows. Instead it's quite common to see black represented by very dark grey, or by a black-to-grey gradient (similar to the video James linked in the airbrush topic). You can also mix in other colours - both the RG and RW would work very well with a slight blue tint - not only is black pigment usually in fact a very dark blue, but shadows and darkness usually have a blue colour to them.
So black is black, but the ways you can represent it vary massively.
There's no such thing as perfect, but you should definitely be pleased - I think he's coming out very well indeed!
Have you ever tried pigment powders for things like dirt?