All Blood Angels possess a genetic flaw which, when breaking a sweat in combat, sometimes causes them to go batshit crazy and forget about the Emprah for a bit, become lost in bloodlust, or even begin hallucinating that they are Sanguinius in his final battle against Horus. Other Space Marine chapters watch the Blood Angels in battle with a mixture of awe, fear, and Codex induced jealousy.
It all essentially harks back to the end of the Horus Heresy, where Horus the arch-traitor laid down the smack-down like no other, killing Sanguinius before getting served by the Emprah. The death of their Primarch essentially traumatized the Blood Angels so hard that it became a genetic flaw. A closely related phenomenon is the marginally less severe Red Thirst, which existed long before the Horus Heresy (possibly as an attempt by Khorne to push the Blood Angels into serving him).
In either case, those marines (and dreadnoughts) who suffer from the Red Thirst and Black Rage get grouped into a special non-codex Death Company, where they get stripped of any iconography and get their armor painted black with red x'es. This force usually is accompanied by a Chaplain, almost entirely focus on close combat with thunder hammers and other Power Weapons, and tend to use Jump Packs. Those unlucky bastards who didn't die in the fight either get chained up to get a Viking Crewcut by Astorath or get shepherded into a tower for them to brood until the next fight where they can be put out of their misery.
Thankfully the Red Thirst and Black Rage are not some kind of contagious space cooties, so other chapters are safe from it. It is certainly not related to Weeaboo Rage when Astartes forces are on the receiving end of Tau weaponry.
However, there have been many attempts to cure it, including Corbulo's neverending search, experimenting with Corvus Corax's research, and FABULOUS Bile's Bloodborn experiments with the Red Grail. However, there are no definitive cures for it, and the only documented people who managed to remain sane after contracting it are Chaplain Lemartes (who was later instated as Guardian of the Lost) and Mephiston, who managed to overcome the Rage altogether to become a badass vampire motherfucker extraordinaire. Rumor also had it that the Lamenters chapter also possessed a cure for the Black Rage within their gene-seed, but considering the fact that they've been shat upon since their conception, the odds of finding a live Lamenter are next-to-impossible and the black rage came back after a while anyway. The Angels Resplendent were able to suppress the Black Rage just by becoming artists. But then they were visited by a "prophet" when all the Librarians and senior members were gone. This "prophet", most likely a daemon, told them they were being retarded and they should embrace the Black Rage, eventually leading them to a civil war before becoming the Angels Penitent, noted for their larger-than-average Death Company.
How to tell if you have the Black Rage
- If you think the enemy commander looks like Horus.
- If you think your battle brothers look a lot like Rogal Dorn and the Emperor.
- If you remember the terrible Feng Shui inside of the Vengeful Spirit.
- If you have an overwhelming urge to spill, shed, or drink the blood of your enemies.
- If you like killing too damn much.
- If you get so angry even an Angry Marine would like to offer you a cold one to CALM THE FUCK DOWN.