Sounds like what you have is a common King Snake or California King Snake (see pic)... They are harmless, and often feed on other snakes including rattlesnakes, hence getting the name King. 

The only poisonous snakes you might run into in AZ are rattlers, sidewinders (which are basically rattlesnakes that move sideways), or the Arizona coral snake (red, yellow and black bands; red and yellow bands touching.)  All other snakes are harmless... And all are beneficial to humans and the ecosystem.

www.snakesofarizona.com ...here's a site to peruse.