Something which happens suddenly and unexpectedly is "out of the blue." Example: "They seemed to be talking calmly when out of the blue she slapped him in the face." It is as if suddenly it started raining on a day when the sky was clear and blue. You are suprised and unprepared because you did not see it coming when it happens "out of the blue." Example: "Why did she do that?" Answer: "I have no idea. It was completely out of the blue."