The Blood Ravens are a Space Marine Chapter that was created for Relic's renowned Warhammer 40,000: Dawn of War RTS game series and subsequently included in Games Workshop's official Warhammer 40,000 source materials as well.
The Blood Ravens are a Chapter organised according to the Codex Astartes standard, although they have an unusually large number of Librarians. The high frequency of psykers among their ranks has resulted in increased testing of the Blood Ravens genetic material that is submitted to the Adeptus Mechanicus, but they are considered genetically stable.
The Chapter is not aware of their earliest history, and remain ignorant of their founding or the identity of their originary Chapter or the lineage of their gene-seed, and thus the ultimate identity of their Primarch. As a result of this, the Blood Ravens' philosophy centres around the quest for knowledge and information, and the Chapter places extreme importance on the recovery and preservation of relics, and the passing down of ancient lore.
Seemingly in despite of this, the Blood Ravens are known to be a highly secretive Chapter, and many of the secrets and relics they unearth quickly become inaccessible to the uninitiated, and this has many times in their history raised the ire of the Inquisition. Indeed, in the past this secretiveness has allowed heresy to take root in the Chapter, though each time the Blood Ravens have vindicated themselves.
There is also a minor mutation in the stage 6 implant (the catalepsian node, a brain implant controlling the Marine's circadian rhythm and sleep functions) of Blood Ravens that results in eidetic memory, but with the inability to experience REM sleep.
The following text contains plot spoilers.
During the events of the game, a force of Blood Ravens numbering at least one squad joined the Liberation Fleet dispatched to Forge World Graia, and shortly after arrival, a Blood Ravens Tactical squad met up with Captain Titus en route to his rendevous with Ultramarines Thunderhawk Lance of Jove. Together they fought and defeated several squads of Chaos Space Marines from the Chosen of Nemeroth warband and their auxilliary Renegade Militia infantrymen. They were then reinforced by an additional Tactical squad of Ultramarines deployed by drop pod, who joined them during the final leg of the journey across a Chaos-held bridge.