From the Super Mario Wiki
A Black Bogmire (also referred to as a Bogmire Shadow) is a weak "clone" of the actual Bogmire, found in the game Luigi's Mansion. They attack Luigi, but Luigi can use them as projectiles to defeat Bogmire. Luigi can suck up a Black Bogmire with his Poltergust, which gets stuck as a ball on its nozzle. Luigi can then aim and blast it at Bogmire to make it vulnerable. After hitting Bogmire with a Black Bogmire, Luigi may need to avoid Black Bogmires while trying to capture Bogmire itself with the Poltergust. They come in two sizes. Small Black Bogmires are about as big as Luigi, whereas large ones are nearly twice his size. The smaller ones are easier to suck up, but the larger ones do a better job of weakening Bogmire, thereby allowing Luigi to deplete more of its health.