Why is Reign of Terror sometimes called a historical film noir?

Reign of Terror (1949), also known as The Black Book, is often categorized as a "historical" film noir because it applies the stylistic and thematic conventions of the film noir genre to a period piece set during the French Revolution.1

While film noir is most frequently associated with contemporary crime dramas of the 1940s and 1950s, critics often use the term to describe films that share a specific visual language—such as the use of deep shadows and stark lighting—and an aura of cynicism. By placing a thriller narrative within the volatile, high-stakes political environment of the French Revolution, the film adopts the dark, suspenseful tone typical of noir while departing from the genre's usual modern-day urban setting.21