So it is not that blood magic itself inherently makes a mage more susceptible to demon possession but more that these mages are willingly consorting with demons putting themselves at risk?
The Chantry claims that it does. I'm uncertain whether that's actually true. Bear in mind that this is a setting where the Chantry by design doesn't have all the answers. (They also claim that Blood Magic de facto causes you to be evil no matter why you use it, which we have from Word Of Gaider is false. On the other hand, it's not like I can't see where they got confused: most Blood Mages seem to indicate that this is true, and nobody can remember that correlation is not causation.)
Edit: Now that I think about it, I remember reading that as per World Of Thedas Blood Magic does tend to thin the Veil, which makes it easier for demons to cross. On the other hand, I don't remember reading whether that's because of the Blood Magic itself (in which case this is innately a bad idea, though there can still be threats bad enough to justify it) or because of the pain and death that blood magic tends to cause (in which case just opening your hand isn't a problem.)