I kind of failed to get the point across (and it was a wall of text so im sure most folks didn't even read it) but.... Blood Magic, the main reason its 'evil' is well, look at the player abilities. 2 out of the 3 are not 'morally grey'. They sacrifice people. And in the lore, all the other blood magic involve sacrificing or enslaving people. Yes at base Blood Magic is just a means to use your blood (or the blood of others) to fuel basic spells instead of mana, ultimately giving you a higher degree of spells per encounter but the 'vast' majority of Blood Magic involves sacrifice.
Circle mages use lyrium to fuel larger spells (which they rarely practice as is), blood mages sacrifice people instead. That's why its bad, that and the whole blood enslaving which isn't even a mind control. You don't make them 'think' its the right thing to do, you take over there body, forcefully and make them do what you want while they're stuck, helpless watching whatever it is you make them do.
Outside of the utmost hedge-mage basics of blood magic (and ignoring the fact its an art learned from demons) there is almost no grey area. It's 99% evil, 1% 'grey-but-definitly not good'. The very fact they allowed players to use it the main reason anyone thinks differently.