It makes perfect sense. The blood elves, having broken from the Alliance, now find themselves in the ragtag collection of misfits that is the horde. I don't know what to make of the Dranei, quite frankly, because they just seem random, but the Blood elves as Horde only bothers me because everyone who is not alliance gets shoved in there (see: the Forsaken) and not because they are not alliance. The alliance treated the blood elves like crap in Frozen Throne, and those elves have started feeding on Demonic energies which makes them quite opposite from the light-serving races. The Horde should be Orcs, Trolls, and maybe Tauren and the others should belong to a different faction. That would have been the way to go with the expansion IMHO, but the two-side thing has been set in stone for a while now.