Not exactly; blood magic apparently has to be taught, there's some trick to the casting like with ordinary spells, it's not as simple as slitting your hand open and flicking the blood at your enemies. In some cases, like Idunna in DA2, they can even use spells like Blood Control without actually shedding any blood (though that could just be bad design/cutscene power, as warrior/rogue Hawke is able to resist the compulsion even without help from a mage, so it must not be that strong). This is speculation on my part, but the repeated bloodletting tends to leave you weakened (i.e. you just lost a lot of life energy to cast those spells), thus the demon who taught you (or any, for that matter, they tend to hang around mages anyway, then they weaken the Veil around them with the blood magic and suddenly they can converse with them) makes a move to enter your body, to which you acquiesce because you're either desperate or literally drained of life and willpower. When a demon possesses a person, they no longer need them to slit their wrists, because the art of raising the dead into an army and binding lesser shades is as natural to them as breathing is to humans. They may even be directly using the mage's life force to power these spells with the blood now a superfluous red liquid.
Spirit healing as a school  follows a similar structure, that there are certain ways of weaving healing magic that only the benevolent spirits know of. While they may be passed on in books and manuals and the like, the means to power these spells often goes beyond the limits of one's mana and lyrium, and requires a benevolent spirit to burn off part of its essence to power the spell. Thus we get lots of people capable of spirit healing in the DA lore, but only those with actual benevolent spirits in their bodies like Wynne and Anders can pull off the most powerful spells. RShepard227 (talk)