I am in the process of restoring my car. I am pretty picky about trying to replicate the factory details in any way possible so I have been looking at hundreds of photographs of low mileage original cars, I have not seen any with black undercoating. It was fairly common in the US for the salesman to try and sell the buyer "Rustproofing" when they sold a car as this was a very high profit add on. Perhaps that is what happened to your car.