A bolt from the blue

Meaning

A complete surprise, like a bolt of lightning from a blue sky.

Origin

This has the feel of a Shakespearian or Biblical phrase, but it isn't as old as it sounds. There are several forms of it: 'out of the blue', 'a bolt out of the blue', etc. The earliest citation is Thomas Carlyle, in The French Revolution, 1837:

"Arrestment, sudden really as a bolt out of the Blue, has hit strange victims."