These spaces are supposed to signify that the room has, at that point, ended. Certain new homes do not contain black spaces.
The majority of furniture items cannot be placed on black spaces, however, certain players have figured out tricks so that they can place items there. These tricks, are known as the 'Black Space Tricks' and there are multiple versions of them.
Eventually, a feature was added that allowed players to turn off black spacing, enabling users to place items anywhere in the rooms, including solid furniture like couches and chairs on the walls.