They were characterized by the higher density of alcohol, stronger and more harmful than conventional illegal liquor such as Doburoku, and also called yami-zake because they were sold in yami-ichi (black markets).
どぶろくなどの従来の密造酒と比べてアルコール濃度が高く、激烈で有害なのが特徴で、闇市で売買されることから闇酒ともいう。