out of the blue

• out of the blue

• out of a clear sky

unexpectedly, from nowhere

From right out of the blue he asked her if she wanted to get married.

(from Idioms in Speech)

a sudden surprise, something quite unexpected

A life, they say, may be considered as a point of light which suddenly appears from nowhere, out of the blue. (R. Aldington)

We were sitting at the supper-table on Carey's last day, when, out of the blue, she spoke. "How would you like to live in London, Jane?" (J. Walsh)

"Well, there's one happily married couple, any way," I used to say, "so congenial, and with that nice apartment, and all. And then, right out of a clear sky, they go and separate." (D. Parker)