Those look like Black Diamond valves to me. The black coating often wears on the stem, particularly at the bottom and to a lesser degree at the top-which is where all valve stems wear. It doesn't seem to hurt anything as the stem itself is supposed be nitrided. The thing to watch for on these valves is transfer of guide material to the stem. If this is the case, both valve and guide should be replaced. If no metal has transferred and you measure clearance, be aware that this assembly needs at least .003" clearance to survive.
The right piston crown indicates that it has had water in it-probably from the crankcase vent. I'd bet that the top ring started to rust to the bore at some point and that the ring broke when the motor was started.