Blood Magic is not the only dark/forbidden magic in the Dragon Age universe, Flemeth's way of Magic as well as Shape Shifting are debunked and forbidden by the Chantry. I would suspect that since Merrill was able to learn a form of blood magic on her own, it suggest that the Dalish utilize forms of magic also forbidden by the Chantry. I don't want to sound as if I'm trying to put words in the mouth of the creators but magic in and of itself is dangerous because of its ability to be abused by people, so any kind of magic can be Dark/Evil but that is subjective based on the actions magic is used in, not by the particular kind of magic (if you want to even categorize it) being used. As has been said some forms of blood magic are sanctioned (phylacteries, the sealing of Coryphaeus, The Joining Ritual is likely the product of blood magic as well,)
What im saying is don't feel that the lack of no Blood Mage specialization means, no alternative, if the story calls for it, they can bring as many dark themed specializations to light as they want, and it wouldn't even be that much of a stretch to write up a few based on the nature of several types of magic already used. If magic was all good and none threatening then the Chantry wouldn't feel the need to cage all mages, rather they'd just seek out those who used Blood Magic if you believed that only Blood Magic is dark and evil.
As far as the on going discussion goes, maybe some of you have read something that suggest that necromancy in the form of animating corpses other than skeletons requires blood magic, if so how about providing some sources or references, if not there doesn't seem to be much ground to base this on. DA2 had the one mage who combined Necromancy and Blood Magic (which he learned from Orsino of all things) in his research to try and construct a living thinking breathing undead construct stitched together to resemble the dead wife he had lost, this isn't simple Necromancy at this point, and nothing suggest a demon or spirit at all was used to animate the leandra corpse amalgam, as it was infact Leandra in control of the mind of the thing. So no i don't think Necromancy requires blood magic, or is evil, but it can be used for evil, it certainly can be used to make a character look more unhinged if not unsavory in the eyes of the masses if they are using it to do some nefarious means.