【out of the blue】
◆【同】out of a clear (blue) sky
・Out of the blue, John asked Susan to marry him. ジョンは、何の前触れもなく突然、スーザンにプロポーズした。
One day, a mother mouse and her child were taking their daily stroll when out of the blue, they were cornered by a cat. Keeping calm, the mother mouse yelled out "Woof! Woof!" Startled, the cat ran off in fear.
The mother mouse then hugged her child and said, "Now you can see why it's so important to learn a foreign language."