Black moors are a type of fancy goldfish. They're black in color and have rounded bodies and protruding eyes. Before you put anything in a tank with your black moor—whether other fish or decorations—you need to think about the possible consequences.
Black moors have a couple of handicaps which you need to allow for. The most obvious is their eyes. Although they look interesting, they make it hard for these fish to see well. Moors' poor eyesight means they take longer to find food. Their eyes are also susceptible to damage. In addition, their rounded bodies mean that they're slower-moving than some other types of fancy goldfish.
Black moors are naturally social fish who will shoal with others of their kind. Therefore the most logical choice is to house them with other black moors. This means they'll all get equal opportunity at feeding time. When deciding how many fish to keep in your aquarium, make sure you allow 20 gallons for the first fish and 10 more gallons for each additional fish. So if you want to keep four fish, you need a 50-gallon tank.
Other Fancy Goldfish
If you don't want to keep a tank of just black moors, your other option is to mix them with other fancy goldfish. These are the only suitable kinds of fish, sharing the same temperature and water requirements. Make sure you house them with other slow-moving fantail goldfish, as singlefin varieties are too fast and boisterous, and could eat all the food before your black moors get to it, or even bully the black moors. You could go one better and house them with other fish that have visual impairments, such as bubble eye goldfish or telescope goldfish. The same space requirements stand as when you're keeping them with other black moors.
While you can put some items in the tank with your black moors to offer them cover, you should avoid any jagged decorations or ornaments with holes for hiding. The bulging eyes of these fish can easily get damaged or even removed by the wrong kinds of decoration. Ideally you should provide them only with plants—either live or made from silk—as many other tank adornments are just too much of a risk.