I've been looking at both on this website and the only major difference I can see if the Black rat snake has black eyes while the Texas rat snake has blue eyes. The head shape is slightly different too. There was a note that the black rat snake doesn't get as big as the texas rat. The only other thing I've heard is the texas rats can be more nippy and harder to handle than the black rats.

http://www.cornsnakes.net/otherprice.php3

As I'm really interested in acquiring one of these species I was wondering if anyone would care to comment on their similarities and differences. I'm currently leaning towards the LTR as I'm so impressed with the ones I've seen on BPNet.