Demons can get to any mage, though. It's just (apparently) easier for demons to get to blood mages, since many blood magic specific spells involve actually invoking spirits.
Regarding Anders, I think his hate-on for blood magic is more of a deflection of his own misdeeds--he allowed himself to become possessed by a spirit, and while it originally wasn't a malicious spirit, his own anger and hatred warped Justice into a demon, Vengeance.
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