During twilight, the light emitted by the sky is of a particular hue of blue for several minutes after the sunset (same happens several minutes before sunrise). This period of time has been termed "blue hour" by painters and photographers. English use The Oxford English Dictionary (OED) says that The earliest known use of the noun blue hour is in the 1870s. OED's earliest evidence for blue hour is from 1875, in Blackwood's Edinburgh Magazine. I found the quote in Blackwood's Edinburgh Magazin...
