In the original age of film noir, the 1940s and 1950s, black and white film stock was used for the crime and thriller genres that film noir encompasses, rather than the colour film used for musicals and other prestige features. Black and white shadows have therefore become a motif of film noir, even in modern film noir that eschews the black and white palette.
Normally, light suffuses a film image. Background and foreground get lit almost as well as the actors. Commercials in particular favour well-lit images. I found a uniformally-lit public service announcement for a Depression-era campaign to raise funds for the unemployed. The presenter is Eddie Cantor, a vaudevillian who successfully switched to Broadway, radio, and big and little screens.
I used HitFilm Express to darken the image. I enjoyed learning about the software, but I make no claims for a great facility in putting filters on the digitized filmstock. Nor can I claim to have used actual low-key lighting. I suspect it's difficult to imitate the actual lighting technque called "low key" simply by adding and removing colours digitally. High-key and low-key have meanings more complex than their two-word terminology implies. My self-assigned task was to alter existing film, not learn how to make a film.
I used the first part of this file: https://archive.org/details/AFP-66AG_A
Below is the altered clip (pseudo low-key):
Eddie Cantor's cheerful voice counteracts the dark tones, I must admit. Nevertheless, the PSA brightness is gone.