Black Gears are exactly what their name implies - black gears. They are sent out by Devimon, embedded into Digimon that are normally good and turn them evil. This is a way for Devimon to get new henchmen to destroy the Digidestined. The Black Gears can be destroyed by a strong enough attack, or by being touched with a Digivice.
More information: Devimon enemy bio