AdmundfortGeographer
Getting lost in fantasy maps
The original core books build off the standard AD&D cosmology, not so much FRs. There is mention of the four elemental planes with the cleric, there is mention of the Positive and Negative planes in the section on Battling Undead in Dark Sun.Dark Sun is not FR, the core books present the FR cosmology as a starter set of assumptions because it hews closest to "classic fantasy" assumptions. The Grey is the Grey, the Black is the Black. I will say I like how 4e incorporated "the lands between the wind" for the feywild and some of the primordial/god lore as possible backgrounds, but Dark Sun is explicitly not a setting that's intended to feature great bouts of planar travel.
I asked Troy about where the inspiration for The Gray and The Black came from as they were not mentioned in the original books. He admitted that cosmology was never considered by the original design. They needed the setting isolated from the rest of the AD&D settings simply to prevent the powerful Dark Sun characters from getting into the other AD&D settings. So voila, planar isolation by game rule necessity.
The Gray and The Black came up later as Troy was writing the novels and he felt he needed to handle an afterlife in some way for the novels (The Gray) and a place where Rajaat was imprisoned that existed outside of the world (The Hollow, inside The Black).