Blood magic is a branch of magic based on the employment of blood to cast spells and perform rituals. It was considered by the Elders to be a form of dark magic, because it can be used to transform an individual into a specific magical being.
In exchange for immortality, the Necromancer, Knansie, made a deal with an upper-level demon to use demon blood in her resurrections. The use of demon blood causes a transmutation in the resurrected individuals, eventually making them become demonic. This eventually led to Macy Vaughn, a witch and Charmed One resurrected when she was a baby, to develop demonic powers, such as the Evil Sight and pyrokinesis.