I think its the Black legion, in the screens and previews i have read and seen it is the black legion, theres no mistaking the black and gold trim on the marines and the eye of Horus symbol. Which makes it more confusing in that apparently Eliphas (comfirmed in PCZone uk) is returning and he is a Dark Aspostle of the Word Bearer legion, so whats he doing with the black legion unless he now serves abaddon.
I do like the idea that you can corrupt the blood ravens through the campaign, i just hope relic keep that in like there were supposed to keep the morale choices in DOW2 which they didn't.