A bolt from the blue
Definition 1 of 1
Pro Tip 1/3Unexpected News
Use it when sharing news that is surprising and not foreseen by anyone involved. 
Her decision to move abroad was a bolt from the blue for her family.
Pro Tip 2/3Emotional Impact
Reflects strong reactions, often shock or disbelief, because of the suddenness of the event. 
The news of the beloved mayor's retirement struck the town like a bolt from the blue.
Pro Tip 3/3Dramatic Reveal
Use it when describing a moment that dramatically changes the situation or people's understanding. 
The unannounced return of the missing painting was a bolt from the blue.