: But you can use mandrake on SB to hurt him
The details of the magics he uses to preserve his form are not known, however there cannot be any doubt that they're Dak magics due to the description we get on how to attain them [self-mutilation and human sacrifice sure don't sound light to me].
And Healing is a light attribute. The effects of Healing and Dark preservation magics reacting together is very much like 2 prescriptions you shouldn't take together, because they react with dangerous effects.
Also, Healing destroys Undead things. Soulblighter isn't truly Undead because he wasn't destroyed, only chased away.
-Welly