It's probably a wandering garter, thamnophis elegans vagrans. Scott Felzer acquired one a few years ago, totally black, no white on the chin like melanistic sirtalis sirtalis.

Adamanteus: Scott Felzer got a melanistic wandering, all black from Montana a few years ago, we have hypermelanistic wanderings in Utah. I have also read about melanistic maritime garters on St George Island.....