Blood has been used for magical purposes for hundreds of years. The most famous adepts of blood sacrifice, especially human sacrifices, were the Aztec and Mayan priests and Celtics druids. Blood is also frequently found in primitive Greek or Roman rituals. Blood is important in magic because it represents life force at its most basic nature. By capturing the evaporating essence of a dying body, a skilled magician can dramatically improve the energy of his spell by linking it to the mana of the body. This augmented energy allows the spell to be more powerful but it is also more demanding on the caster because he must control the fabrics of the spell and tie it simultaneously to the mana of the dying spirit.
However, for such a feat to be possible, the magician most have an inner comprehension of the fabrics of the astral space, thus making this kind of ritual only available to initiates. Another important component of Blood Magic is faith in the results it can produce. The sacrifice can't carry more power that the caster thinks it will carry. In Astral Space, the will and the emotions makes the magic happen, if you think sacrifices are just abject things you can't get any effects to help your magic.
Anyway, for those who can use it, blood can help in several ways in the magical process :
First, it can enhance the power of any spell that can be cast in a ritual manner
Second, it can help in summoning some spirits and even free spirits
Lastly, it allows the caster to use some specific ritual magic, which requires blood as a component