The name Demansia atra is actually invalid. It was previously used however D. vestigiata is now the current name. (See for eg A. Greer's Biology and evolution of Australian Snakes)
Black Whip Snake...while general usage is not a great name....due to variance in colour...some are black...see photo in Rick Shine's Australian Snakes - Natural History, then look at a pic in Mirtschin's Book.
While not recorded in Demansia vestigiata rapid colour change has also been reported on the genus.