I have quite the colony of mantella's, let see, I have 8 viridis, 7 goldens, 5 laevigata, 1 pulchra, 1 madagascariensis, and 2 crocea. The viridis are definitely the most "hidden" species, but they do come out quite often. My males will usually come out of their bromeliads, and call, while the females only hide when they see me around the tank. The others are very active frogs, especially the goldens, and the laevigata's. I definitely agree that their terrarium has to do with their hiding habits. If you have them too crowded, they will be stressed out, and hide a lot, or if you don't have it set up very well, they might hide. So, just keep these things in mind when dealing with mantella's, because I have seen no reason to classify any of them as "hard to see frogs".
As for the d. azureus, does anyone know any good breeders. I emailed some people that have had them available in the classifieds here, but no one will email me back, so, I'd prefer to find a good breeder that will answer me. Also, if you have any tips on setting up their vivarium, I would appreciate that too. I am probably going to give them a 20 gallon size, for 2 to 3 frogs. THanks again,