Can your NPC mages become Blood Mages if you are not a mage? (Spoiler)
Solution 1:
Yes and no. You will not be able to unlock the specialization on that character, but the specializations are actually designed to unlock across playthroughs, so if you get the Blood Mage specialization on one Mage-class character, you can then use it on any further mages - either the Player Character, Morrigan, or Wynne - on any other playthroughs.