Limitations on who the Soulblight can affect?

Are there any fixed rules or general theories about who would be immune? Obviously daemons and undead wouldn't work, and I guess trying to turn a stormcast would just get you a mouth full of lightning. What about Chaos? Are souls already sworn to Gods out of reach? Can it only affect humans? Any spooky fyreslayers running around? Ogors? Gargants?!

Thoughts and theories welcome!