So... Vampire is like Blue Mage... But it's actually not like a Vampire at all; it doesn't suck blood, or die in sunlight, or even sparkle. So... why not just Blue Mage? Why bring "Vampire" into it in the first place?

We already have Dark Knight to appeal to folks looking for edgy dark darkness of the dark. And if Dark Knight just isn't enough, Blue Mage can certainly be edgy and dark without being a Vampire. Just look what they did with it in FFXI! Holy crap!