Portrait in Black (1960) (Blu-ray)

An adulterous couple (Turner and Quinn) conspire to kill her tyrannical husband. Soon after the murder, the lovers in crime receive an anonymous letter that says only: Dear Mrs. Cabot: Congratulations on the success of your murder. With this alarming complication, their new life together begins unraveling as they become trapped in the realization that someone somewhere knows their dark secret.