Blood Magic is a form of magic that, as its name suggests, is intimately intertwined with blood. The most frequent form of Blood Magic is rituals where blood is shed in order to power up - or provide the fuel for - standard spells. Depending on whether Blood Magic is being presented as a neutral art or an evil one, either a mere sacrifice of blood by carefully cutting or piercing the skin will suffice, or only the sacrifice of animals or sapient beings will work. Of course, sometimes they exist side by side, where you can cut yourself (or an ally) for relatively "gray" magic, but vastly more powerful spells can be wrought by ritually killing people, and this is considered black magic.
This style of magic is often associated with the Necromancer, Warlock and Conjurer. A rarer sub-form of blood magic, usually seen in /v/ media rather than /tg/ media, actually uses blood as a physical medium to attack foes with, typically combining an edgier form of water elementalism with life draining