Black triangle

Black triangles are a type of UFO that look like, well, black triangles with or without lights on their tips. The type seems to have become popular in the 1980s and 1990s. By some coincidence, this is about the same time when vaguely triangular stealth aircraft like B-2 and F-117 were tested and commissioned. Some crazier individuals might propose that these stealth aircraft are actually reverse engineered black triangles. For some time, sightings of black triangles became much more common. Either the old flying saucers became passé for alien pilots, or this is just a new fad in hoaxes.

Explanation
Most likely there's no single reason explaining all cases. Nevertheless, here are several possible reasons (assuming that the observer actually saw something triangular): One particular declassified document from the UK ministry of defense suggests that most sightings could be a sort of buoyant plasma, that is to say a completely natural phenomenon.
 * Any group of three lights in the dark sky may be interpreted by an observer as a single, triangular craft, even if it's actually three aircraft flying in a formation or something entirely else. Given that a number of these sightings occur at night, this is probably one of the most plausible explanations.
 * Misidentified stealth aircraft: the military would be very happy indeed to have their secret stuff "identified" as an alien craft Unidentified Flying Object, although some claims overlap with conspiracy theories such as that about a top secret plane called the TR-3B which is capable of flying into space.
 * A "stealth blimp" (see http://www.thestealthblimp.com/, though this one borders on a conspiracy theory)
 * A trio of first or second generation Naval Ocean Surveillance System (NOSS) satellites — US spy satellites that fly in a triangular formation.