Bolt from the blue

A bolt from or out of the blue, is a complete surprise, something unexpected. The allusion is to a thunderbolt from a bright blue sky, which of course would be a very unusual and quite unexpected event. The first citation of the phrase is Thomas Carlyle The French Revolution (1837) who wrote, “Arrestment, sudden really as a bolt out of the blue, has hit strange victims.” Blue meaning the sky has been in use since the 17th century. See also Bolt.