Re: [Spoilers All] Are there any physical consequences to the use of blood magic?

Not so much lore based but more human based will be my answer.

Blood magic in game is easy power.  Meaning it requires less study, less mental fortitude, and offers greater sources of power for harvest and use...ie blood of yourself or others.  This then makes it easy for lazy and more evil leaning people to have power.  If an evil person could spend thousands of hours working to have a skill or just cut some mooks throat, which are they likely to choose??  A good person wouldn't consider the second option so they HAVE to put in the hours to learn or have the skill.  So in many ways, it will almost always be used more by evil than good, simply because it is socially unacceptable, and an easy source of power if you don't mind going against society and harming others.