Although the name “Sith” is now used by the order of Force users who call upon the dark side, the term originally referred to a Force-sensitive species of red-skinned humanoids. The term “Sith pureblood” is now used to distinguish between those descended from the species and the order itself.
The Sith species was discovered three thousand years ago on Korriban by exiled human Dark Jedi fleeing Republic space after their defeat in a war called the Second Great Schism. The Dark Jedi quickly enslaved the primitive culture, and over many generations, the two groups intermingled.
Eventually, the elite ruling class of Korriban consisted almost exclusively of the hybridized offspring of the Dark Jedi and the high priests of the Sith people. These offspring tended to exhibit the physical characteristics of their Sith parentage, such as red skin, yellow eyes, bony protrusions on the face or head, and fleshy tendrils dangling from the cheeks and chin. For a brief time, Sith blood was seen as a sign of weakness‒but the modern Empire believes purity of heritage carries the strength of the Force.
Genetically speaking, true “pureblood” Sith are virtually extinct, but red skin is still the mark of an Imperial‒almost always a Force user‒who can trace his or her ancestry into antiquity.