[itsme]

Looks like it may be nitrided. Its a super thin surface hardening. Helps with wear. I would ask the guys that sent you them what they coated it with. Nothing to worry about and I wouldn't take it off. I have some cams that have that black coating on the lobes where there is no lift. We have a Masarati low mileage engine at work and it's cams are still completely black.