Well, Salamanders have jet black skin, though that is fue to the planet most of them are raised on (Nocturne).
Edit: Just wondering, do necrons count? You know, rusty, obsidian, alabaster, that sort of thing?
Sometimes i wonder 'Why is that frisbee getting bigger?
', and then it hits me.
If the Imperium found us as we are now, they'd probably declare exterminatus on us purely because of Justin Bieber. Wouldn't want that taint to spread any further than one planet.
The Imperium is crumbling because they're getting attacked from all angles. Why did the Eldar empire crumble? Because they got bored.