Well,
I have no explanation but I would like to add that I met a lady walking around Repticon Atlanta a couple years back. She was carrying a similar looking snake. It was just a normal but it was so black you almost couldn't see the pattern in it, sort of like an adult Black Rat Snake. I asked her what it was and she said she purchased it as a normal and a year or two later it started to change and then just kept getting darker. Until I saw this thread I hadn't seen anything else like it. It's interesting that only a couple have shown the trait and the animal throwing it doesn't show any sign of it. Was it the same pairing that produced all of the subjects?