The Black Silk Stockings (1981)
aka Les bas de soie noire
aka Educating Tricia
Patricia’s mother puts her daughter in the hands of Vincent, a married man who shares his life between his piano and lovely maids, in order to give her an education (a sex education). Jeannette, the maid, puts Patricia on the right track by having her witness the hot parties given by Vincent, from behind a see-through mirror. On these events, the women are dressed in black underwear. Things continue in that way until one evening when Patricia, dressed in black, gets through the mirror to join them.