In order to be considered domesticated, an animal really has to be altered in temperment and behavior, at least a bit, from its wild ancestors. It also typically shows color/pattern changes. BPs only show color/pattern changes so far. Part of the reason they have not yet been domesticated is that people are not too fussed over breeding for temperment and behavior, yet. If a snake eats well, and it's a female, it's likely to be bred by most breeders.

Over time, this will change. Ball pythons will be selected for larger adult size/clutch sizes in females, docile temperment, and reliable feeding behavior. Once these traits are consistant in captive populations, I think it would be fair to consider them domesticated. The real tipping point will be when imported wild ball pythons become undesireable, because their behavior and appearance is not as predictable and not conducive to being a pet.

There are probably several species which should be considered domesticated, but currently are not:
Budgerigars and cockatiels, leopard geckos, and golden hamsters, for example.

There are also some domesticated species people don't commonly think of as domesticated: Ferrets, society finches, and koi fish, for instance.