Well, this might be another way in which we may say that the Templars are using blood magic besides the philacteries.
But by that logic, we can say that all magic is blood magic. After all, mages are people with a high amount of lyrium in their own blood, lyrium that allows them to call upon the powers of the Fade. Same for the Templars, they can only use their skills once the lyrium in their blood has reached a high enough value that it can help the caster use his mojo.
We also know that mages also use lyrium or lyrium potions to increase their powers or do more complicated rituals or spells. As I see magic, the lyrium in a mage’s blood, it’s like a mini-battery of sorts, but as a biological component, like some sort of blue blood cells, besides the red and white blood cells present in any human being. When a mage uses mana the normal way, the lyrium cells discharge their mana load, and they can’t be used again until they are charged again, either by the passage of time, or by imbibing a potion.
Blood magic ups the ante and uses the entire ‘lyrium blood cell’ in the process, destroying it completely, but giving the user a much greater momentary power boost, at the cost of having to wait longer for those lost cells to be recreated and then recharged. Well, that goes for the user using his own blood, I guess there’s no problem if he uses the blood of his enemy/just an unlucky victim.
And I believe there is another drawback to blood magic, but this is just a wild theory with no proof other than everybody seems to know ‘blood magic is bad’, but can’t explain why that is.
Well, my theory is that using blood magic(as in completely destroying the lyrium blood cells) continuously is a big no no because when the destroyed lyrium blood cells are replenished in the bloodstream, some of the lyrium used in an unnatural way turns to red lyrium. As a matter of fact, this might actually be how the red lyrium was created in the first place. And if the mage is using the blood of another, some of the ‘taint’ might still be transmited to his own blood, in the magical backlash of the unnatural use of magic.
So, the more blood magic someone uses, the more red lyrium he has in his blood, thus making him more susceptible to outside influences. Not to mention completely bonkers, taking a page from Belatrix Lestrange’s book of ’How to go cray cray by using too much dark magic’. .