The black carpet 4 speed thing is a mystery that I have been following lately.
My 69 4 speed Dart has a blue interior with factory installed black carpets.
I had thought it was a factory mistake, but have been told by others that the black carpet is correct with the 4 speed.
Since then I have seen some other 4 speed cars with colored interiors and black carpets. Both A and B-Bodies and I believe that I also seen a E-Body that way.

Not a lot of cars today still have their factory installed carpet sets. I assume that when a carpet set is replaced that most people are buying one that matches the interior color.