Debra Winger was given the choice of the two roles in Black Widow (1987); she chose the role of the FBI agent, because she didn't understand the motivation as to why the Black Widow kills, so the title role went to Theresa Russell.
The film refers to deaths being caused by "Ondine's Curse" which website Wikipedia define as a "congenital central hypoventilation syndrome (CCHS) or primary alveolar hypoventilation. CCHS is a respiratory disorder that is fatal if untreated. People afflicted with Ondine's curse classically suffer from respiratory arrest during sleep".
The film's two top-billed name-above-the-title actresses, Debra Winger and Theresa Russell, both had multiple character names, they having two and four respectively. Winger was Jessie Bates and Alexandra Barnes whilst Russell was Renee Walker, Marielle Dumers, Margaret McCrory and Catherine Petersen.
Many movie posters for the film featured a long text preamble that read: "Every man she marries is mysteriously murdered. One woman has uncovered the truth. But proving it could be fatal". This thriller film noir actually debuted around six months prior to the same 1987 year's big box-office suspenser hit Fatal Attraction (1987).
The "black widow" of the film's title is often considered a synonym for a "femme fatale" the latter of which which is defined by the Wikipedia website as "a stock character of a mysterious and seductive woman whose charms ensnare her lovers, often leading them into compromising, dangerous, and deadly situations", like a black widow spider. The literal definition of a "black widow" spider refers to a species of widow spiders known as "Latrodectus" which is a genus of spider in the "Theridiidae" family.
According to "The '80s Movie Rewind" website there is an alternate ending to the film. "Theresa Russell goes to jail, Debra Winger walks out of the building, that's all. The other times . . . it has ended with . . . Debra Winger sitting on a tropical beach having a drink, wearing a big sunhat...".