bolt from the blue, a

a bolt from the blue

Something unexpected or surprising. We always thought of Michael as a life-long bachelor, so it certainly was a bolt from the blue when he returned from his vacation sporting a wedding ring!
See also: blue, bolt

bolt from the blue, a

Also, a bolt out of the blue. A sudden, unexpected event. For example, Bill's dropping his life insurance was a bolt from the blue for his wife. This metaphoric term alludes to totally unforeseen lightning or thunder from a cloudless (blue) sky. [First half of 1800s]
See also: bolt