One word: values.
If you're working in black-and-white (graphite/grayscale), use at least 5 tones of shading -- ranging from dark (but not pressyourpenciluntilitsdead dark) to light (minimal pressure). In color, using blues and purples (mentioned above) works well.
Check out Ric Druet
on deviantART.com -- he does a lot of drawings involving Fresians (obviously black horses). You can get some inspiration from him. (: He's also very polite, and I'm sure he would answer your questions if you asked him for a few tips.