There are a couple of really dark tartans woven by Locharron. Look for the "Dark Isle" tartan and also the "Highland Granite" tartan.

Dark Isle is almost black, the tartan is fairly subtle...or at least it looks like it, from the pictures I've seen. Highland Granite is more of a real tartan in dark greys and black. It's very handsome. If I had unlimited funds I'd get a kilt in Highland Granite.