A blood doll is a human favored by a vampire as an entity worth preserving. When the vampire visits a blood doll it does not try to kill or sire him, but instead withdraws a portion of blood. After doing so, it waits until the person has had time to recover, and then pays him another visit. The human experiences strong sexual pleasure from these visitations. By employing blood dolls, a vampire can satisfy its need for blood without leaving behind a trail of bodies that lead hunters straight to it. If a blood doll is a particularly worthy follower, the vampire might also gift the human with a portion of vampire blood, forestalling the human's aging to some degree.
Like all relationships in vampiredom, a blood doll only exists as long as the vampire feels it is useful. If a blood doll ceases to be useful (and useful could mean tasty or loyal), the vampire will probably kill him rather than deal with the security threat the doll's existence represents.