I just thought, if nobody has pointed you at Steven Bol's website you should take a look because it's probably the best source of information about Mexican garter species (and your obscurus most probably comes from one of his breeding group). Steven brought most (if not all) of the Mexican species to Europe and provides great information on his site, which I think is all translated from Dutch now.

Thamnophis eques obscurus - Steven Bol Garter Snakes