The very nature of the blood magic requires that equal parts be used in spell casting. Blood Mages often wear battle regalia inlaid with small spikes that dig into their skin to let small rivers of blood drop to their fingertips. Once drained, they use their schooling to twist and turn that life force into wicked burning flames or powerful magics of living fire. Practicing mages rarely engage in battle since the magic is so draining and dangerous, but the more the Blood Mage is willing to sacrifice themselves, the more powerful their casting will become. Apprentice mages use the blood of smaller creatures, and attempt to turn their life force into the wicked flames that are signature to the caste. The three to four spheres that circle a Blood Mage are constant living flames that are empowered by the psyche of the magister. They can be used to manipulate into wildfire that will leap from their fingertips. Apprentice magicians need intense concentration in order to keep these verdant spheres from extinguishing, but as the magician grows stronger their base subconscious master it on its own. After prolonged use of blood magic, the magisters are infused with it fully. They have the ability to control all of their body and turn themselves into living avatars of flame and destruction. –Blood Mage Thadious, ‘Inner Workings of the Court’
I don’t think that blood mages actually use blood in their magic. I always thought it was akin to the name “blood elf”, where the mage is simply showing pride in being Sin’dorei and “blood” is how they do it. The spells largely seem to be arcane based, so why would they need some odd, profane ritual to use it?
The Blood Mages in Blasted Lands use the blood of animals to rebuild the Demon Hunter. There are a few other examples I can’t think of off the top of my head but they send you out to collect blood for them.