The blue hour is the period of twilight when the Sun is at a significant depth below the horizon. During this time, the remaining sunlight takes on a mostly blue shade. This shade differs from the colour of the sky on a clear day, which is caused by Rayleigh scattering.
Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal
Die blaue Stunde (1890), painting by Max Klinger
Twilight (1897), painting by Edmund Tull (Ödön Tull)
Taj-Mahal, twilight (1920), woodcut by Charles W. Bartlett
Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this illumination occurs.
Midtown Manhattan during civil twilight, demonstrating blue hour
Civil twilight in a small town in the Mojave Desert
Evening nautical twilight on Lake Ontario, Canada
Long exposure of nautical twilight in a small town in the Mojave Desert