out of the blue
Definition from Wiktionary, the free dictionary
Short for out of the clear blue sky, likening a sudden and unexpected event to something unexpectedly falling out of the sky.
- (idiomatic) unexpectedly; without warning or preparation
- After I hadn’t heard from her in six months, she called me out of the blue to meet for lunch.
- I really can't understand how something like this could simply pop up out of the blue.