“Every mage can feel the dark lure of blood magic. Originally learned from demons, these dark rites tap into the power of blood, converting life into mana and giving the mage command over the minds of others. Such power comes with a price, though; a blood mage must sacrifice his/her own health, or the health of allies, to fuel these abilities.” – Dragon Age: Origins, Blood Mage specialization description
Blood magic is a form of magic that uses the power inherent in blood to fuel spellcasting. To put it simply, Blood Magic is the ability to use HP to cast skills instead of MP. This essentially increases the battler’s effective MP.