These marks are located on upper surface of blades, and behind the leading edge protection....I didn't have the occasion to look at them with a magnifier myself.When I talk about protrective coating is actually the paint layers.
I don't think it's coming from a particular engine exhaust since it affects several helicopters. Although we have experienced stormy weather recently I don't have the feeling it comes from direct lightning hit, and beside this as Nick states a lightning strike wouldn't have stayed unoticed to crews....
Sorry, my position here doesn't allow me to send pictures and it could be misinterpreted by managment.