Blood hunters are also known as lambs, and are not as uncommon as normal dnd. Pious people in usually sacrifice goods or food or pray in order to show affection and gain favour of the gods, but it is possible to be super dramatic and even offer your own blood (or someone elses, depending on the deity).

Of course, this has varied results, but Lambs/Blood Hunters have narrowed down the blood offerings to a science, even to the point of being able to actively use it in combat situations. Thus, they are mostly a religious class in this setting, akin to clerics or paladins :) Some deities dislike this form of worship, others favor it. Same with different temples or cultures.