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 were meant to attack Luigi, but Luigi could use the ghoul as a projectile to defeat Bogmire. Luigi could suck up a Black Bogmire with his Poltergust, which would get stuck as a ball on its nozzle. Luigi could then aim and blast it at the true fiend to make the ghost vulnerable. Unfortunately, Luigi had to watch out for other Black Bogmires when trying to capture the real Bogmire 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 the ghost's health.