Black Rock is a location near the Great Valley. Like its name suggests, it is a large black-coloured rock. It is slightly into the Mysterious Beyond. It is featured prominently in "The Hermit of Black Rock" where it is revealed to contain a large cave system.
In the episode, Petrie and Guido were dragged out to Black Rock strong winds when they flew (or in Guido's case glided) too high. They then fell into the caves, where it was revealed Petrie had injured his wing and Guido had injured his foot meaning they couldn't fly/glide out. After wandering for a while, Petrie and Guido found a bush of Ground Stars to eat from (Guido eating the Crawlers living on the bush). There they met Swooper, the titular hermit. Swooper later flew out with them on his back, and despite him being blind and never having flown for many years, he succeeded in taking them back to the Great Valley.