Bolt from the Blue
Aparecer de la nada
This expression is used to explain that something shocking happened suddenly, unexpectedly.
This expression was first used in the 1800’s. When sky is blue, it means that everything is going fine. There is nothing to worry about. But if on that calm peaceful day, a bolt or lighting appeared unexpectedly. It would break the peace and calm we were previously enjoying. Now, let us imagine that in any given day, something surprising came to disrupt our normal life. Then, we would be able to say that that shocinkg event came just like a bolt from the blue.
Principal: What happened here? What did you do to your teacher? Why is he lying on the floor?
Students: We don´t know. We were just having our class; the teacher said, «Let me show you this…» And suddenly, out the blue, the teacher put his hands on his chest, he shouted, «Ahhhh!» And he just fell down.