The pureblood siths are in no way elevated position compared to the rest as far as I know.
The first dark jedi, who renounced jedi ways and were driven away by the Jedi order, were not Sith. They arrived to Korriban after being exiled from galactic republic and used force to enslave the Sith race that lived on the planet. They became 'Sith lords'. I believe dark jedi were humans originally.
In time, the species intermingled. They became known not as a race, but as Sith culture.