User blog comment:B214/Chapter 504: Rift/@comment-4945830-20161002203645/@comment-7418318-20161003051744

Maybe Silver himself was immune to this soul corruption because his soul was no longer his own, but Keyes'. And if the source from which he learned his magic was incomplete, he could very well have been unaware of the side-effects.

I doubt he'd have given Gray this power without telling him about the possible repurcussions and the advice to use it as seldomly as possible if he had been aware of the dire consequences.