noun
blue-line area; illegal prostitution zone
Historical term for areas marked with a blue line on police maps where illegal prostitution occurred, roughly from the 1940s until the 1956 Anti-Prostitution Law. Often contrasted with 赤線区域 (red-line district).
See also: 青線地帯
In postwar Tokyo, there were many places called blue-line areas.
青線区域は、警察の地図で青い線で囲まれていたことからその名がついた。
The name 'blue-line area' comes from the fact that these areas were enclosed by a blue line on police maps.