Day for night is the name of a cinematographic technique to simulate a night scene. Mainly intended to avoid costly (and technically challenging) night filming, outside scenes can instead be shot during the day, with special blue filters and under-exposed film to create the illusion of darkness or moonlight.
These two images were taken about 60 seconds apart. The right one is underexposed about 2.5 stops. Shooting underexposed, rather than using a 'blue filter' is much more natural, as light sources can 'punch' through the blue with their natural tungsten color. If a filter were used, this would tint the light sources.