I've never heard of this snake before, but apparently they are pretty common in Texas. Among other things, they enjoy eating rattlesnakes; they are supposedly immune to their venom. They are harmless to humans though.

Wiki Link: http://en.wikipedia.org/wiki/Drymarchon_melanurus_erebennus

From the web: If you see one of these don't kill it. They are pretty common in S.Texas and during deer season many out of state hunters shoot them not knowing what they are.

The Texas Indigo also called the Blue Indigo are the longest snakes in N. America and can exceed 10 feet, so when you see one in the field it is an impressive site, but what is even more impressive is the fact that they are imune to rattlesnake venom and rattlesnakes are a staple of their diet.

Most ranches will warn huntersnot to shoot them and I've seen signs in gas stations and restaurant in
S. Texas with pictures and explanations as to what these snakes are so that they don't get shot. There is a saying in Texas, "If it's an Indigo, let it go."