You might need to do some selective breeding for that. I would suggest catching a few from your area and pairing them up this Spring. Keep the hatchlings that you think will have the most potential. That will of course involve guess work, however since all the adults in your area look similar it won't be as inconsistent as breeding animals from different areas, which is what I do. I have found you can pretty much tell which hatchling will have the darkest ventrals as adults. If a hatchling has the underside of the neck all white, and you don't start seeing any mottling until close to mid body, that will not darken. Keep the ones that have the most heavy checkering or mottling as close to the chin as possible. I have made progress with that in my breeding project, but have still not gotten solid black all the way to the first ventral. IMHO it will be a much taller order getting the chin to blacken, however you might start watching for labial makings upper and lower,( kind of like what kingsnakes have), and working with that. Anyway hope this helps.
Alan,