Source: http://monsterhunter.wikia.com/wiki/Black_Diablos

Black Diablos are a female-only Subspecies of Diablos introduced in Monster Hunter G.

The change in color is meant to act as a warning to other monsters, as she becomes extremely aggressive during this state. This aggression translates into her combat approach, with a significant boost of speed and power becoming highly evident.

Highly aggressive, even for such a large wyvern. Diablos are considered one of the most aggressive creatures in the Desert. Using the sand as cover, Diablos can launch devastating ambushes on intruders approaching their territory. Black Diablos behave like a normal Diablos while fighting. They are highly territorial wyverns, they will attack members of their own species if they enter their territory, and they will also attack other monsters like the Tigrex should they approach.

Commonly found in Deserts. Black Diablos are well suited to arid climates, but their ability to tunnel in sand may limit them to deserts only. However in Monster Hunter 3 Ultimate, they can be found at the Deserted Island during times of extreme heat and drought in the Sandy Plains.