Does anyone else want some backstory on this one? Black grass sounds like something caused by a lovecraftian horror.
It's simply a description following a fire of some sort, a not unusual occurrence in the summer months on hills and mountains.
On the other hand "......in the foothills of the brooding Black mountains lay a meadow where many ghostly apparitions had occurred, a meadow of black grass........"
Cue the creepy music...