How do you become a blood mage on dragon age

In the Arl of Redcliffe quests, you will need to find a way to free his son from a demon. As a mage character (it must be the Warden, not an ally) you can enter the fade to confront the demon, and one dialogue option is to bargain with it. Unlocking the Blood Mage specialization is one reward it will offer.
If you have the expansion, Dragon Age: Awakening, you can buy a manual Blood Mage in the tavern in the city of Amaranthine, while you search for Kristoff.