I have been looking at the picture for a while. I was inclined to think it was a cross between a Black and a Gray, but I think that Duffy's first impression is correct. There is a good chance that someone lost their pet Texas Rat a few years back. (I think you would need to find more like yours to confirm that there was a Texas Rat breading in the area.) It could also be the case that it was a breeder's "science experiment" that got loose.
Exotics are everywhere in the South. I remember seeing Mediterranean Geckos on my window screens at night when I was spending a summer doing research down in Pensacola a few years back. (I could have sworn that I wasn't in Europe that summer.)
I was wondering if readers of this forum had pictures of Black x Texas intergrades or Black x Gray. That might shed some light on whether this is a natural Black Rat morph or just an intergrade.
Thanks for posting the photo and telling us about your find.