Black Coating on internals
 

I broke a tooth off the upper gear in my Bravo 1.when we disassembled the drive for repair there was a black coating on all of the internal parts.This black would not come off no matter what we used. It did not seem to have any negative effect on any thing as the wear was minimal.I purchased the boat new and have always used Merc gear lube plus. Does anyone have any idea what caused this?