I have a few varieties of blackberry that have been fruiting for me here outside of Hilo at 600-700' altitude. I decided to give them a try based upon a guy a short way down the coast (at similar altitude) that was successfully fruiting Natchez. I'd need to check which ones are fruiting for me. Correspondence with a prof at University of Arkansas put Natchez at 300-400 chill hours so I selected clones that were the same or less. I see Navaho listed as 800 chill hours so I would try one with less if I were you.
John