Blood Mage (Origins)

A blood mage is a mage who employs dark rites, originally taught to mortals by demons, in order to access more powerful magic. A blood mage must be willing to sacrifice his own life force or that of others in order to wield this power. Blood mage is one of the mage specializations in Dragon Age: Origins and Dragon Age: Origins - Awakening

Blood Magic in Thedas
Blood magic was the first form of magic on Thedas. According to legend, it was taught to Archon Thalsian, founder of the Tevinter Imperium, by Dumat, The Old God of Silence. Historians argue on this point, suggesting the Imperium's mages may have learned it from the elves of Arlathan. Whatever its origin, Blood Magic was used by the magisters of the Tevinter Imperium to rule over the whole Thedas. According to the Chantry, its reckless use eventually led to the blackening of the Golden City, the creation of the darkspawn and the first Blight.

In the contemporary world, blood magic is described as being one of the more "sinister" types of magic. Blood magic is the magical practice of using blood, life itself, as a potent fuel for magical spells. This life may be supplied by the mage, or by willing or unwilling sacrifices. It lets the mage control the minds of others as well as use their very own life force to fuel their power. This practice is so rare in Thedas now that it can now only be learned from contacting a demon, with the risk of becoming an abomination.

The Chantry strictly forbids the usage of blood magic as it supposedly eventually leads to corruption. Mages using blood magic are labeled maleficars and hunted by the Order of Templars, an order created for the purpose of controlling mages and, more specifically, for hunting down maleficars and apostates. Indeed, the Chantry seems almost irrational in its fear of Blood Magic; going so far as to suppress anatomical study and ignoring more immediate or severe threats. The Circle of Magi endeavors to supervise all magic-sensitive people from a young age and ensure none of them tap into this forbidden school of the arcane. However, even their constant supervision allows the occasional maleficar to slip through their fingers and out into the world.

Blood Magic and the Grey Wardens
Despite blood magic being banned in Ferelden, the Grey Wardens occasionally use it as a means to fight the darkspawn. This is confirmed if the Warden speaks with Duncan on the topic during the Mage Origin.

In the Warden's Keep DLC, an ancient Grey Warden mage called Avernus has utilized blood magic to manipulate the darkspawn taint used in the Joining. He even goes so far as to torture his fellow Wardens in order to learn the secret powers found within the blood while theorizing the extensive capabilities it holds and stating that only within the ranks of the Grey Wardens can blood magic be used freely and allowed to flourish in the name of arcane progress. What he learns allows him to use blood magic to survive unnaturally for hundreds of years as well as discover powerful talents and spells that the Warden can choose to use to unlock the Power of Blood school. Avernus argues that the Chantry foolishly forbids blood magic because of their belief that it caused the corruption of the Golden City (which he dismisses as lies) and that there is a great deal that the Grey Wardens, and mages in general, can learn from it.

Blood Mage Items
(Obtained by completing four on the side proving matches in Orzammar, given by the proving armsmaster.) (Looted from Avernus at the end of the Warden's Keep DLC) (Inside an egg in Wyrmling Lair during Urn of Sacred Ashes Quest) (Located in an armoire during the Freedom for Anders quest in the Awakening expansion)

Notable Blood Mages

 * The Magisters of the Imperium
 * Jowan
 * Avernus
 * Caladrius
 * Zathrian
 * Mad Hermit
 * The Baroness
 * Uldred (though it is also possible that he only sought the help of blood mages, and was not a blood mage himself)

Links
Blood Magic Guide on Gamefaqs