OK, here is a picture... sorry my car is bit dirty right now
I can take a better one when it is detailed if you need it.
It is an undercoating. I could not find any places that the coating is scratched to see if it is that color throughout, but I did chip a very small piece out around the jack hole (the undercoating is much thinner there) and it appears the undercoating itself is a light gray. I suspect the dark gray color is some sort of elastic paint on top of that.
My car is original as it left the factory/dealer; never any paint/body work. The underside of the car is the same as the rockers.