The Black Morass is the area now known as the Blasted Lands, where the orcs first appeared in Azeroth through the Dark Portal.
The Black Morass was once a wetland marsh much like the Swamp of Sorrows, but when the Dark Portal was opened, the dark energies of Draenor began sucking the life from the region, leaving it a dry wasteland.
It will be available as an in-game region with the expansion, accessible through the Caverns of Time.