What is a blood alliance?

Blood alliance has two meanings, one is the ancient Chinese meaning of taking a blood oath. As in the answers above. The other is somewhat related to superstition. It refers to a secret party organization of 7 clans of vampires, also known as the Blood Alliance. (Because the vampires call themselves the Blood Clan, the Clans consider themselves to be the most civilized of the Blood Clans, and therefore call their alliance the Blood Alliance.) . The details here would be too complicated. Perhaps a separate wikipedia article is in order.