I had this idea last week, and meant to put it up before now, but obviously I got lazy. Another fluff idea, but no need for a paint scheme, they are still World Eaters, just with a bloody circle as decoration where possible.
Even the mindless violence that is the Blood God of Chaos occasionally needs specific targets removed with little collateral damage. Whether its the removal of an enemy leader so that one more in line with Khorne's wishes ascends, or preparing a future champion for a life of hatred and bloodshed by destroying all they love before their eyes, the potential is endless. The average Berserker is ill suited for this role. Likewise, the daemons of Khorne are not known for their ability to reign in their bloodlust once the primary target has been removed. Thus, the Circle of Blood.
This warband follows signs and omens across the galaxy, making surgical strikes and small raids as their lord demands. Their name comes from a bizarre effect upon the battlefield: as the World Eaters prepare for the charge, a large circle of blood appears close to the person they are there to kill. The World Eaters make for this unnatural landmark and slaughter all around it, throwing the bodies into the circle. Once the being that Khorne has targeted has been placed in the circle, it vanishes, taking the corpses with it, to feed the daemonic legions. Once the circle has gone, the Khornate marines take care to leave the battlefield, inflicting as few casualties as possible, for none know why the Blood God aimed them there, just that he did.
While the average guardsman or space marine might not be able to tell the difference between the typical Khorne Berserker and a Circle of Blood Berserker, there are differences in the training, indoctrination, biological modification, and (most importantly) worship practices. The Circle's warriors veiw Khorne as the patron of the application of military force. Of course, this does not preclude them from enjoying the slaughter, just that they tend to have more control.