'Out of the blue' is an idiomatic expression describing something that seemed to happen or occur out of nowhere unexpectedly. This expression is similar in meaning to 'out of thin air'.

1. I hadn't heard from Gary in years, but yesterday he called me totally out of the blue.
2. I turned the corner in the car and a deer ran onto the road out of the blue.
3. I don't like spontaneity so I hate when plans change or something comes up out of the blue.

1. I didn't plan to go into finance, but an opportunity appeared out of the blue and I decided to take it.
2. Shirley was a great boss, so it was sad for all the staff when she announced out of the blue that she was leaving.