I've always assumed fading was primarily from the sun, but others have pointed out that often the fading seems to have occurred equally on the bottom. If it was strictly from the sun, it would seem the bottom would be as faded unless the bowls were left outside upside-down or something.
At the same time, I was always under the impression that the INSIDE tended to stay black, which suggests the sun must be involved in the fading.
I just picked up an old patent pending 18" black with transitional wheels that is faded (unles they made them in grey), so I'll get to check out the inside out myself after I clean the gook, but I suspect it will be a much cripser black.