black market adult video; underground porn video
Refers to illegally produced or distributed adult videos, often sold through underground channels.
He was arrested by the police for buying black market adult videos.
bootleg video; pirated video
Can refer to any illegally copied or distributed video, not necessarily adult content. Less common than the adult video sense.
Bootleg copies of that movie are circulating online.
Compound of 裏 (ura, 'back, reverse, underground') and ビデオ (bideo, 'video'), literally 'underground video'.