I've been working on my CSM army, as well. At least planning it out and such.
The basic fluff is that this is a force of chaos which worships and follows the orders of Be'lakor. While he isn't "helping" Abaddon do evil things, he is finding subversive ways to strengthen his warband. The pursuit of daemonic artifacts, relic vehicles, gene seeds, and other goodies has made them a force to be reckoned with.
The army contains some elements of all four gods, as is befitting to Be'lakor's history and stature, but for the most part this is an undivided force. Plenty of marines, cultists, helbrutes, aircraft, and so on. Plenty of sorcerers, and a few chaos lords as well.
They have been known to make alliances and cooperate with other chaotic armies to accomplish mutual goals, or deceive them into doing the heavy lifting before stabbing them in the back and robbing them blind. They follow Be'lakor, what did they expect? There have been recent reports of Be'lakor's forces attacking an Imperial forge world alongside the Brass Stampede Khorne Daemonkin legion...
Here is the basic color scheme:
And their sigil, which I think I can pull off freehanding:
I had a CSM primed black, so I decided to test out the paint scheme. I LOVE black and purple in combination and I've really been looking forward to painting some CSM in these colors.
Quick and dirty Vallejo Black and GW Xereus Purple:
Pretty happy with it.
Then I cleaned up the purple and washed the mini with GW Druchii Violet:
I like the effect very much. It's quite subtle. The purple is deepened a little bit, and the black has a very thin tint of purple as well as some very faint purple edge outlines.