The Ichorians are so named because they protect the holy blood or ichor of the Chosen. It is for this reason that, in Vulgate, they are called the Bloodguard.

All Ichorians have half of their body—right or left—tattooed black. They are divided into two forces of roughly 30, 000 each: the Red Ichorians and the Sinistral Ichorians.
The Red or Dextral Ichorians belong to the Great and are commanded by He-who-goes-before, who the Great elect in their Clave. All Red Ichorians have the right side of their bodies tattooed. They are also distinguished by wearing service collars of gold. They form two legions each identified by a sigil: the red lily or the pomegranate. Unlike the other legions of the Commonwealth, the two Ichorians possess a full complement of 27 huimur each. Both legions are stationed in the Canyon of the Three Gates, whose gates or fortresses it is their duty to defend.
The Great use the Red Ichorians to maintain their part of the Great Balance. By controlling the Canyon, the Great keep the God Emperor hostage, making certain that he does not gain direct command of the other legions of the Commonwealth.

The Sinistral Ichorians, in contrast to the Red, have the left side of their bodies black with tattoos. Also their service collars are made of silver rather than gold. They are commanded directly by the God Emperor and are restricted to the Isle. Their purpose is to protect the God Emperor and the rest of the House of the Masks against incursions by the Great.