I think it was just not wanting to impliment anything to make blood mages significant. "we don't want to have to put in stuff to make blood magic feel significant, so lets just add a necromancer tree (which is freaking blood magic anyway!)
Mike said in an interview they wanted to make specializations significant and affect the story.. I was REALLY hoping that this desire would come to fruition. :|
Im sure they hav told you before, but Necromancy is NOT Blood Magic.
Blood magic relates mostly to how you extract the power to create magic, taking it from blood. You can cast fireballs with blood magic and be still a very evil spell. Not because you casted a fireball, but because you sacrified blood or maybe a life to do it.
Just remember the circle of magi in DAO. You can use a ton of lirium and mages to enter the fade, or you can sacrifice a live with blood magic to do it. The spell is the same, but the way you do it, its what defines Blood Magic.
DA2 did a lot of harm to blood magic's lore. They really messed it up.