From Longman Dictionary of Contemporary Englishout of the blueout of the blueinformalEXPECT if something happens out of the blue, it is very unexpected → a bolt from/out of the blue at bolt1(3) → blue
Examples from the Corpus
out of the blue• Out of the blue, he asked me to come with him to Europe.• Symptoms of the disease often appear out of the blue.• They just came around, out of the blue, to try and hurt me.• And now here was a shiny new fence, built out of the blue while our backs were turned.• Do you remember Jane? Well, she phoned me yesterday, completely out of the blue.• Then, too, his thinking had hardly dropped out of the blue.• One evening, Angela phoned me out of the blue and said she was in some kind of trouble.• It came to me out of the blue.• She told me, out of the blue, that she was going to live in New York.• It was totally out of the blue.• The pair are travelling in their caravan when a sinister family pitches up out of the blue.• Now you turn up out of the blue talking about us like we were a Lionel Ritchie lyric.• Even with a mysterious ex-lover who had turned up out of the blue after more than sixteen years' absence. 