Demon Clean Engine cleaner was chemicaly developed not to etch any metal or turn any metal used in model engines black, regardless of how long it was left on the engine.
As far as I know, it is the only thing that has ever been on the market that works this way. Even anti freexe in a crockpot will turn an engine dark dingey gray if its done improperly.
I do not have a suggestion on how to chemicaly restore the looks of your engine since the surface was literaly etched. The tumbler method as described above may work.