out of the blue

(idiom) without warning

Example Sentences:

I was sitting at home, reading the newspaper, when out of the blue my best friend from high-school called. I haven’t talked to her in 20 years!

Out of the blue, the policeman ran into our office and began to chase the criminal.

Similar idioms:

1. out of nowhere
2. blue in the face
3. once in a blue moon
4. blue
5. cut to the chase