Here on the Island
T. sirtalis fitchi will have a grayish-blue to black underside. Which I have three of.

T. elegans usually are gray-brown or black, with a dark, checkered pattern between yellow stripes. And I have one which I consider het melanistic due to the fact that several years ago, before my interest in garters my cat caught a melanistic and had it trapped in and around where my dry vent leads out side. The snake I now have, his belly is a very dark blue to black and his checkered pattern has a rich bluish black mixed in.

T. ordinoides are very common here I have found out here are often red and orange on the back, but I have two with the red stripe and they have this bluish-black belly.
Conclusion “black belly” very common here.