Jackplate Coloration

My jackplate is 10 yrs old and waterlines over the years have discolored it of course..Bought some of the Eagleone Etching Mag cleaner and it worked great, brought back the chrome/white look again...My question is : the last 2 trips Ive went on, after pulling the boat out of the water, my jackplate is damn near black (looks similar to an electrical burn) around the water line.. whats causing this? My boat hull is white and there is not much of a water line on it, but on the jackplate it looks horrible where the waterline is...