Black gnats are small insects with dark coloration, although they are rarely truly black. There are small flies people might call black gnats, including black flies or even phorid flies.
A likely insect called a black gnat would be dark-winged fungus gnats. Fungus gnats, as their name indicates, breed in fungus. Indoors, a common source of fungus would be in house plants that have been overwatered. The gnats are in small numbers are not directly harmful to the plant but can cause root damage when present in significant numbers. For most homeowners, however, they are nuisances as they fly about the homeowner’s face and land in drinks.
If homeowners see “black gnats,” they should contact a pest control professional who can determine what actual species they have. The professional then can partner with the customer to develop a solution.