Conventions Of A Film Noir

Classic film noir developed during and after World War II, taking advantage of the post-war ambience of anxiety, pessimism, and suspicion. It was a style of black and white American films that first evolved in the 1940s, became prominent in the post-war era, and lasted in a classic "Golden Age" period until about 1960 (marked by the 'last' film of the classic film noir era, Orson Welles'.

Film noir is not a genre, but rather the mood, style, point-of-view, or tone of a film. It is also helpful to realize that 'film noir' usually refers to a distinct historical period of film history - the decade of film-making after World War II, similar to the German Expressionism or the French New Wave periods. However, it was labeled as such only after the classic period - early noir film-makers didn't even use the film designation (as they would the labels "western" or "musical"), and were not conscious that their films would be labeled noirs.
Primary Characteristics and Conventions of Film Noir: Themes and Styles.
The primary moods of classic film noir were melancholy, alienation, bleakness, disillusionment, disenchantment, pessimism, ambiguity, moral corruption, evil, guilt, desperation and paranoia.