How is Blood magic evil? Both the warden and Hawke can become blood mages and they are not evil.
Blood magic isn't evil, it's just using it makes it easier to become evil. It's like guns in the real world; guns aren't evil, though the people using them can be
The main problems with blood magic in DA:O and DA2 were:
1. Too weak - Mind Control was very weak spell in DA2 due to asymetric combat system, and the fact that there were usually ~20 enemies on screen, controlling one was useless + there was no option to use it anywhere else than combat. Main appeal of blood magic is therefore lost.
2. While I could stomach Meredith ignoring mage Hawke and his companions because of Champion thing or something, being a Blood Mage should result in being immediately executed in a templar-controlled city.
3. No-one ever commented on Hawke/Warden being a Blood Mage. Wynne, Alistair, Leliana, templars, Anders, Meredith, Elthina, Sebastian etc. were all blissfully ignorant of the fact, that Hawke was the most powerful Blood Mage in the city.
4. Blood magic is very risky too use, because it can tear the Veil - especially if it's already thin like in Kirkwall.
Now think about the last point in context of events in DA:I. Inquisitor sealing breaches in the Veil can hardly use blood magic that can easily tear it.