Someone who is 100% of a certain nationality. Someone could be a Greek pureblood, Bulgarian pureblood, Russian pureblood, Mexican pureblood, etc.
Sofia: Yeah me and the rest of my family are Greek purebloods.
Nina: No way! Im a Bulgarian pureblood!
Vlad: Im a Russian pureblood!
Steve: Well this is awkward....im not a pureblood