The Blue Caste were an order of sorcerers from the East. Little was known about them in the West, except that they had joined Saurons's cause in the War of the Ring against the orders of their supposed leader, Yirokhsar the Blue, and they were the true head behind the Khundolar invasion, them having corrupted and influenced the Easterling's warrior-chiefs.
The Blue Caste was itself split into two factions, the Brotherhood of the Mind and the Brotherhood of the Spirit.
Speculations[]
It had been speculated that the Blue Caste was one of the mysterious eastern cults and/or magic traditions inspired or formed by the Ithryn Luin, and that even Yirokhsar might be the alias of either Rómestamo or Morinehtar, or at least one of their followers or pupils.