Pocket Dimensions opened with the use of a Rift Blade or a Dimensional Door will be entirely black. The only thing the player will be able to see is the portal they just came through. The dimension is actually a cube surrounded by Fabric of Reality blocks. These blocks are perfect black, the same color as the void. It is possible to break the Fabric of Reality with your hand, or to replace it with blocks from the player's inventory. They give off a decently bright light level, making it very easy to see something placed inside the room.
Dimensions entered through a naturally spawned portal can be hallways or rooms made of stone brick, often with a handful of other Dimensional Doors or Warp Doors. Some dimensions have traps or hidden chests.
Player-made dimensions, those made by placing a door in a non rift spot, are completely empty and do not contain Monoliths or dungeons. They can be useful for storing things should one find themselves in need of extra space, or one could link the dimension with another spot in the overworld to cut travel time. However, dieing in such a dimension may still send the player to Limbo.
Some Pocket Dimensions have a dungeon in them. This may consist of several rooms with hostile mob spawners. Pocket Dimensions opened with a Rift Blade or Dimensional Door will never have a dungeon in them.