It may depend on year, but rocker panels for my 1500 midget (75-79) were originally black. I painted them Ford Oxford White, along with some other changes, so mine is most definitely not original. However, one thing to add is that the rockers aren't the only thing painted, as I recall. The black isn't painted straight across the rocker, but curves up toward the front of the car, and the paint may be elsewhere on the car (like behind the rear wheel well). I don't remember precisely, but the cars with black are probably original.
Matt