Blood Magic is also called ‘Sacrifice Magic’, which is probably more accurate. The idea is simple, since magical energy is processed life energy, then causing someone to lose this energy can be a source of power. Ironically, even using some of your own blood can intentionally release more energy then you could normally access. It’s not normally easy to use this power, but the more you try the more you attune yourself to this energy.
However, it’s not much of a stretch between using your own blood and beginning to cut open others for power. Then it eventually happens: You kill something or someone and realize death releases large outflows of magic. The magic that usually flows into Ley Lines over time. Then you learn that the way a creature dies effects how much energy they release and how usable it is. Unfortunately the longer and more drawn out the death is the more usable magic one can get. And so the road to torture begins.
This is why no one sane wants to make use of Blood Magic except in critical situations. Some believe even knowing how this works is dangerous. However, it’s just so easy to find this out that it’s completely impractical to be unaware of the potential of Blood Magic for very long. So instead most magic wielders are taught the dangers of it and breaking the ban on Blood Magic is punished harshly.