Purebloods were Sontarans who had been manipulated by the Rutan Host to destroy Sontara in the 26th century. The Rutans discovered them on a long lost colony world at the edge of the Mutter's Spiral and were able to deceive them into believing that cloned Sontarans had, by their very nature, muddied the Sontaran genetic pool.
Unlike the Sontaran clones, the Purebloods reproduced naturally. However, they still clearly resembled cloned Sontarans, except that they had long hair and individual physiognomies. (COMIC: Pureblood)