The story
Picture a perfect summer day, with not a cloud in sight. Then lightning cracks down from a clear blue sky. Nothing could be less expected, and that is the whole picture behind the phrase.
The Roman poet Horace described just such a moment some two thousand years ago. In one of his Odes he tells how Jupiter, who usually splits the clouds with his lightning, drove his thundering horses and chariot across a clear sky. Horace had brushed off the gods for years; this, he says with a wink, set him straight.
English poets were already calling the sky “the blue” by the 1600s. In 1642 Henry More wrote of “the empty Blew.”
The full phrase arrived with the Scottish writer Thomas Carlyle. In The French Revolution, published in 1837, he wrote of events “sudden really as a bolt out of the Blue.” Readers loved it, and it has meant a total surprise ever since.
