Blood Magic

Blood magic is a rarely used type of magic, and potentially powerful form too. Using the blood drawn from the living or freshly dead, the user can draw upon the life essence of the donor in place of magical components and/or magical foci.
Usually banned in many socities, a number of individuals still practise it, and regularly found in more traditionalist settings. The Abrahamic traditions & nations have pretty much banned such magics citing the belief that the blood is the life mentioned in the Torah, Bible & Koran while many in societies place a ban on magic in general, or at least specific trends.
This has left it to be used in many anamist, shamanistic & pagan traditions, along with it being used in underground sects. The former regularly would use it as sacrifices to specific gods, or their general pantheons, and would rely more on animals, and the odd time, criminals or enemy captives. The latter sects are usually cults or individuals dedicated to or trained by dark & evil deities & supernatural creatures.
Certainly it has been used by conjurers & necromancers more readily than any other type of spellcaster. Both reason that the veil between life on other planes of existence or the life's blood of others can restore the dead or at least contact them, using fresh blood. Many draw the distinction too that powerful creatures rely on the spilling of blood or the absorption of it to sustain themselves, with the vampiric being the most well documented.
It is thought that the blood also helps ground the summoned and the risen by the mystical life properties of the blood itself. Thus, those that use it prefer to use the blood of the living or recently living to conduct the casting or ritualistic sides of spellcasting. 
This has led to a good number of casters (usually of the darker arts) to find refuge or alliances with cults such as the Necrocult or the Children of the Nazarene, and potentially the Order of the Gash. Such asters end up learning unique spells involving the spilling, collecting amd/or manipulation of blood to act as the foci or component of their spells. Many of them are quite willing to kill to retrieve it if necessary.
At the same time, many make pacts with specific ancient deities such as Seth, Lolth, Morrigan and Leviathan to learn this. Creatures such as ancient vampires and their ilk are willing to share the same knowledge with many trading a part of themselves to Samael, Lilith, and similar to gain the same.
Many of these creatures always has it in their favour and willing to completely dominate the caster when the latter has learned enough to further their masters agenda and not their own.

Manifestation

According to many religious lore, blood has always had an important trait. Most will mention in some way or the other that the blood is the life or the life's blood, and it's spilling is highly regulated. In many it is completely off-limits to use in any shape or form even for sustenance.
Thus the blood magician steps in. Willing to draw the blood forth from a source and using it to make sigils & marks on their own body, or even from themselves they wll draw forth the mystic source of life to boost their spells. Sometimes it involves the marking of an area deemed for spell casting purposes, sometimes being ingested, or drawn forth. Whatever the method the blood is vitally important.
When done right it will act as the main source of power for the caster or to augment their own power further. This can be draining and result in the death of the donor if they are still alive as they are drained of life essence.
Most of the time it is powerful enough for the spell to work and usually used to draw forth some type of spirit. This can range from the dead to elementals, to angels & demons, to even deities. Because of it's nature it is likely to condemn the caster and corrupt the area they cast in leaving a magical signature behind, and regluarly one of darkness.
Many casters or areas they have performed rituals using blood will end up cursed due to other higher powers intervening, drawn to the effect of the magic being corrupted with the life force of a living being. It takes an exceptionally strong will, and maybe one not quite sane, to do such magic. Many who do regularly end up dying early especially when not experienced enough or prepared enough for it.

Type

Metaphysical, Arcane