Angels Sanguine
One of the Second Founding successor chapters of the Blood Angels, the Angels Sanguine have a long and proud history in the Imperium, stretching back ten thousand years to the time of the Great Crusade. Indeed, the original Angels Sanguine were members of the Blood Angels Legion at the time, forming the new chapter when the Legion was broken down into thousand man Chapters.
Like most successor chapters, they don't have much fluff written about them, GW leaving the door open on that so players can make their own history. Whether this will happen or not is unknown since they have a difficult colour scheme (dividing one half red, one half black) so unless you have a steady hand and want a challenge you will most likely choose an easier chapter to paint.
What is known about them in the fluff is that they never take their helmets off in public or even to other Space Marines, even from the Blood Angels. If they do forgo their armour, they wear deep hoods to obscure their faces. This has led to a number of rumours about them and what goes on in their chapter-monastery, which has some pretty deep catacombs it seems...
In one of the Blood Angels books, a group of them freak out at the thought of Mephiston looking into their brother's flesh with the warp to see if he was dead. What they were going to do if he said that he had, isn't brought up.
They have also done some secret missions in the Jericho Reach, and they are implied to be responsible for offing a group of Inquisitors who got too nosy for their own good...