Axanthics lack Xanthin, which is yellow pigment. They don't lack the color brown, which can be the result of tyrosinase and melanin. Many axanthics look fantastically black and white as hatchlings, but brown out as they age.
If a T- ball python that is not amelanistic ever turns up, it could be combined with an axanthic to make a truly black and white (and grey) snake.