necromancer has replace blood magic as the dark specialization its dark but not dark enough to the point where people want you dead for being a blood mage ,companions also know your specialization no matter what cole tells your hurting spirits by using necromancer, simply put it has been replace by necromancer cause no body would deal with the herald being a blood mage.
That simply isn't a good reason.
1) Your companions can hate you as much as they want. The majority would follow you either way. It's not like having Iron Bull, for instance, makes or breaks anything.
2) Not many people know you. Heck, more times than not I hear "Is that the inquisitor?". You are a mere story told to the common people.
3) When high ranked mages can barely identify blood magic, how can random joe?
4) Cullen used to be a templar and Cassandra is a seeker. Easy scapegoat there -- they'd hold you in check. About 5 minutes of dialog and it's fixed.
5) People followed Hawke, who may or may not be a blood mage. People rallied behind the grey Warden, who may or may not be a Blood Mage.
6) Blood Magic in itself isn't evil -- it's just a one way road to corruption and becoming evil.
7) You got a point. Necromancer seem just as evil as Blood Magic. Why do people follow a Necromancer?
8 ) Mages are being blamed for the conclave. Elves are being blamed for everything. Why does people follow my Elven Mage in general? Because they don't know he's a mage.
It's explained why blood magic isn't in the game. Blood Magic has a major effect on a character and is also described to be the ultimate power. In Dragon Age Inquisition they wanted specializations to feel important. Hence they did not have the time to add the depth they wanted, and thus did not add it.
I find it baffling. "Nobody would follow a blood mage!" is the go to argument. Why? It makes no sense!
Nobody would follow a mage at this point.
Nobody would follow a random dude with a mark.
Nobody would follow the one dude that opens and closes rifts.
Nobody would follow a Reaver.
Nobody would follow a Necromancer.
Nobody would follow an Assassin.
No human would follow a Qunari Mercenary leading a rebel army
No human would follow a Dalish Elf from the wild to death.
Yet... they do. Why?
About nobody knows the inquisitor. S/he is a rumor.
About nobody knows how s/he fights.
About nobody sees you fight.
Your own army hardly knows you.
You only use your army twice. The last time they follow Cullen, not you.