Are you talking generic Black+Metallic CSM or some of the more colorful schemes?
Black with metal edging is pretty straightforward and, especially with lighter tabards for contrast, looks good and menacing for a typical Black Legion.
For more chaosy Chaos, it's hard to beat the EC/Slaanesh colors. Pink and black is a spiffy contrast that still ties in with the other dark forces. For a really nice take see In the M's Kvlt forces on CMON:
link
Word Bearers in crimson and black another good "sinister" look. Unfortunately, red armies are *super* common in 40k and unless your put scrolls, books, and candles on everything, it will tend to blur with Khorne stuff.
The Sons of Malice scheme is pretty neato, but a lot of painters struggle with white. Also, kind of makes them look like police CSM. Check out this album for some inspiration: link
brings back some oldies.
