It's only a retcon for those who didn't want it to be evil.
I base my determination of a thing by the manner in which is it ultimately used - and in two games I have yet to meet a blood mage that is not a self-serving, wretched (at best) individual who's too weak and craven to do anything but resort to self-mutilation. That includes Zathrian, Jowan, Avernus, Uldred, Merrill, Quentin and Orsino.
So - for myself - there's no retcon in saying it was always supposed to be evil. I think it was presented exactly how it was meant.
This is your opinion, which doesn't contradict anything I argue about, it's in a completely different league all together. You argue morals: Blood magic is evil, people who use it self-serving, wretched, weak. Ok, fair enough, but my point is something else.
What I say is - blood magic doesn't have any inbuilt negative consequences for the user, THAT is a retcon I am speaking about. The Camaron Lee said that blood magic has to draw with itself some kind of negative consequences, which is not true. If a person is content with hurting people to gain power, if a person is content with being an outcast among andrastians, if a person studied blood magic correctly and knows what they are doing, everything should be fine. They will dominate the minds of other, they will use powerful destructive magic and they will be successful at it and be happy with their results. It weakens veil? Any magic does. It attracts demons? Any magic does. It doesn't mess with your mind, you don't become mentally unstable due to using blood magic, the fact that a lot of mentally unstable people use it is a different matter.
Basically what Cameron said came across to me as something like: ''crime always pays'':
1) Which is not the truth, many crimes go unpunished and successful.
2) It's subjective. It's evil in your eyes, not evil in mine, perfectly acceptable behavior.