The Bloodmage is a variant spellcaster that has been around in various editions of D&D in various versions. It is not a core class though I do believe it has appeared in some "official" splat-books. The basic concept is of a mage that either sacrifices part of themselves or others for more power.
Why would I like to play a Bloodmage? I think walking a fine line between the desire to gain more power and still being a hero would be an interesting exercise in role-playing. How far would the character go? What lines would he not cross? It would be fun to explore such a character.