It's a Vampiri#From Dracula to Buffy... and all creatures of the night in between. lineage. basically from their first ancestor to the present, there blood has not been mixed with human blood.
let me put it into other words. the first Kuran to present, has never been involved sexually with a human. if te have read the manga, it shows how juri and haruka are siblings, and that is how they have kept pureblood status.
