The spilling of blood is a potent force in the working of magic. It may be a token sacrifice, but it may also be the loss of life the fuels the spell. Expect mages who practice Blood Magic to be portrayed as evil, or at least grey.
Some blood may be indicated to be more powerful than others. Common types are human blood, the blood of royalty, the blood of a special line, a child's blood, or virgin's blood
. Sometimes only a single person's blood has power, and any other blood is powerless.
The other side of the coin is menstrual blood, which is used much less frequently but is associated with life magic when it is used.
- Melisandre of A Song of Ice and Fire uses blood magic to cause the deaths of Robb Stark, Joffrey Baratheon, and Balon Grey Joy. At least, that's the way it appears. The maegi Mirri Maz Duur also uses blood magic to "heal" Drogo and the favor is returned by Deanerys when she burns the maegi alive as part of the magic to awaken her dragons.
- Mercedes Lackey loves this concept, and blood mages are frequently villains in her books.
- In Stardust, the witches use the hearts of living stars to prolong their youth as a form of blood magic.
- In Death: The Time of Your Life, menstrual blood is used to power a spell to send the characters to the land of the dead and come back alive ( mostly).