These were developed by Dan Coffey (not the beverage!) via selective breeding. I've linked a couple of pics of snakes owned by Dan that I photographed a few years ago. Clearly there is some Black Gap background with these, but not sure what else has been added to the mix over the years. The more colorful of the two snakes shown here is what I think of as "Coffey line" snakes.

SierraHerps.com