I actually liked Blood Magic in the 'Dragon-Age-that-shall-not-be-named' (Which I felt was much more fun, for me at least, than Origins). I only ended up using two spells: Grave Robber, which was extremely helpful since it ended up replenishing quite a bit of health in the middle of a fight, particularly when upgraded, and Hemorrhage, which was absolutely devastating against most of what I fought.
Another benefit of blood magic was the fact that it rendered the cost of sustained spells, like Arcane Shield and Elemental Weapons moot, meaning I rarely had to worry about being unable to cast spells. Added to it that I always used Voracity by the end, along with Awakened's Might, I usually got a little more bang for my blood buck (Literally, since I was also very fond of Fire Magic).
As for justifying it as Hawke, I always felt that, after a certain point, my Hawke was willing to go to extreme lengths to protect those she cared for: Her Brother, her friends, her love, and the city of Kirkwall as a whole, and since she was both too smart and too stubborn to give into a demon willingly, she would see Blood Magic as a tool like any other, particularly since she lived by Malcolm Hawke's Philosophy: "My magic will serve that which is best in me, not that which is most base.".
Not only that, but she wants to prove that even Blood Magic, when properly used by someone who knows the risks and is willing to use it responsibly and for the good of others, has its place.