OP means blood magic, not some sort of blood-bending or what have you.
Like another anon said, for me blood magic is a way to power spells. It's easier in the sense than you don't need practice and study to do magic like this, however dangerous, painful, draining and takes time. It's a relic of primeval magic, of rituals and shamans.
A blood mage is often scarred by the cuts he needs to practice to draw blood, but generally hardier due to being used to the toll such magic has on the body. Chants and gestures accompany casting, and thus it's slow to deploy in combat. Any mage has a limited capacity of spells he can power himself before tiring out from blood loss; therefore, many prepare preserved blood vials, flasks and water skins, or have assistants nearby to take blood from.
Due to the powerful nature of blood, and the fact it's literally the life essence of living being, it's one of the most powerful reagents, and most blood magic is ritualistic in nature. A mage's blood is the most powerful substance.
Blood magic is frowned upon or downright banned, due to the negative reputation it has accrued for being barbaric, primitive, violent and "gorey", as well as being risky and used by many unscrupulous people as a shortcut to magic. Demons prize blood, human blood the most, and it's the simplest way to summon one, and the common form of payment (blood after all carries a bit of the soul and life force of the individual); another reason blood magic is taboo, for careless casting has the habit of attracting unwanted attention.