Blood Magic allows you to use HP to cast spells instead of MP. And trust me, you're going to need a LOT of HP (and some HP Regen and Leech) to be able to efficiently cast your spells. Oh, and MP regen and leech are replaced with their HP counterparts.
And on an irrelevant side note, I've leveled Dracmage to 74 recently. I've also switched from a skull build to a minion build after getting the Necrotic Grimoire. Every bit helps, especially with gauntlet weapons like Master's Grip with the Arcane element.