In various fictional fantasy worlds "blood druids" are druids who engage various forms of blood ritual, such as ritual sacrifice, ceremonial bloodletting, consumption of human and animal blood, and various forms of hematomancy.
In the real world, the practices of the Celtic priests known to history as druids are obscured by the mists of time. Although there was no known group of druids ever known specifically as "blood druids," Roman writers such as the historian Diodorus Siculus did record that the druids practiced human and animal sacrifice and read the future in the gushing blood from the victims.
I'm not sure exactly how true these third and fourth hand Roman reports are, but they are not implausible. But suffice to say, if you are looking for druids, these are not the druids you're looking for!