Instructions
Play the game normally to go through the quest lines and gain experience.
Add points to the magic attribute when you level up. The strength factor that the game engine uses for Arcane Warriors is the magic attribute, not the strength attribute. Putting points into strength is useless for the character-type.
Play through either the Redcliffe or Elven Alienage questlines to unlock the option to become a blood mage. At the end of the Redcliffe quest, choose to go into the fade rather than kill the child to get the blood mage specialization. At the end of the Elven Alienage quest, spare the wizard's life to learn the specialization.