Well, I've always tried to base my characters around WC2 or WC3 units (at least, my non-main, who I base more around racials). When it comes to the Blood Mage, though, I'm not quite sure.
Obviously, they've all got history as Mages, Flamestrike being a staple of the Blood Mage. However, there are also obvious ties with the Warlock's fel magic, not only in Banish, but the overall color schemes. All Blood Mages, no doubt, began their careers as Mages, but so did most Warlocks, so perhaps the Blood Mage is simply a Pet-less Destro Warlock?
Sure, it's not an overly important topic, but it's at least fun to talk about. Plus, as I said, I like to have characters that translate as pre-WoW units.