To answer your question, are they different? Yes they are, but since they show no color, you can't easily tell.
I have read some reports that no leucistic Black Rat Snakes have ever been found in the wild. On the other hand, leucistic Texas Rat Snakes are well documented. Take that for what it is worth.
Since a leucistic snake shows no color, the easiest way to tell would be to cross it with a wild type and see it the offsprind look normal. If you were to cross a leucistic Texas rat with a wild type Texas Rat, the offspring should all look like normal Texas Rats.
I hope that helps you with what you want to know.