Take a small part off of the gun that is CCH and has turned black.
A couple screws, or the forend latch plate if it has one.
Try cleaning just these small part(s) with something like acetone and see if the black disappears and the Colors re-appear underneath.
Acetone shouldn't harm any real case hardening colors.
It will cut through any lacquer or most any clear coat finish applied over it.
Maybe just that clear coat has turned color for some reason.
This will let you find out w/o disassembling the entire gun or having to take it/send it somewhere to have someone do a super-sonic cleaning.
