The reserve where I saw this frog is a known location for Bromeliohyla melacaena, and this frog looks to me like a dead ringer for the photo of Bromeliohyla melacaena in Köhler's Amphibians of Central America. But I have not completely ruled out the possibility that it is an unusual-looking specimen of the much more common species Ptychohyla hypomykter instead.

Here is a list of all the reptiles and frogs I saw on this trip to Guatemala.