What is "black tern"

The black tern (Chlidonias niger or Chlidonias nigra) is a small tern generally found in or near inland water in Europe and North America. As its name suggests, it has predominantly dark plumage. In some lights it can appear blue in the breeding season, hence the old English name "blue darr". The genus name is from Ancient Greek khelidonios, "swallow-like", from khelidon, " swallow". The species name is from Latin niger "shining black".