Black Triangles are much like their real world counterpart. Similar to regular UFOs in the game, Black Triangles are seen an extreme distance away from the player in the skybox. When sighted, they are usually stationary but there have been reports of moving triangles. On online reports, they are frequently called motherships due to their immense size.
Black Triangles can be spotted all throughout Liberty City, but the most sightings come when the player is far out at sea, especially in northern Alderney.
There is an explanation that can debunk many sightings of Black Triangles and UFOs in general. To make the game world feel alive, the developers added large airliners and helicopters in the skies. Although it is rare, it is possible for three of these aircraft to spawn in a triangular formation, giving the illusion of a large UFO.
Other reports of smaller Black Triangles can be explained as faults in the game's rendering or the user's hardware, causing certain polygons in game to distort and turn black. Under the right circumstances, this can create a large solid black triangular shape in the sky, easily mistaken for a UFO.