Has anyone else seen noticeable color-darkening with very old snakes? Recently, I've noticed that Houdini is significantly darker than he used to be. It's uniform all over (it's like his black/charcoal areas are expanding a bit, not like everything is dulled). At first I panicked about a retained shed or something like that, but after a couple months, I can safely say that is not the case.
In fact, he's gaining back all the weight he lost during mating season, and doing well. Just curious if anyone has noticed anything like this?

Again; this is like a uniform expansion of dark areas on his pattern. The bright areas are not dull, and it is definitely not a skin issue. It's not big at all (my husband who sees him every day can't see it). I just thought it was interesting, and wondered if it was common at all.