Well looking at them should make it pretty easy to distinguish between the two.
Just because the common names are similar, it doesn't mean that the snakes look similar.
I don't think there are any black subspecies of Arizona, so telling the difference should be very easy.
Below is a picture of Arizona elegans (not sure of the subspecies). Compare it to the picture at the top of the page of a Purple-glossed Snake
Picture is from
http://www.californiaherps.com
"I am dying by inches from not having anybody to talk to about insects." - Charles Darwin