I strongly recommend you get in touch with Steve Emerick at Napa Valley Snakes. I have a pair that I bought from him 2 years ago as hatchlings, and already they are 4 feet long and 99.9% jet black. Hopefully in a couple years I'll be able to engage in a bit of shameless self-promotion rather than referring people to Steve all the time.
All that's left of their juvenile coloration is a bit of grey on their lower jaws, and a few flecks of red under their necks. I'm sure they'll be gone in a matter of months. He has black milks from both the Mark Bell and San Antonio Zoo lines, so you can get unrelated pairs if you're interested in breeding them.
Nate
Napa Valley Snakes