Like a lightning strike from a clear blue sky - very unexpected! We also say, "like a bolt from the blue". (ie. a bolt of lightning)

It comes from an expression "like a bolt (of lightning) from the blue (clear sky)" You don't expect lightning to strike from a cloudless sky. The expression got shortened over time to simply "out of the blue". A total surprise.

It means 'totally unexpectedly'. I hadn't heard from John for more than 10 years, then I get a phone call 'out of the blue'. It's a nice phrase, and it's also used a lot by native speakers. Do you have a similar idiom in Portuguese?

"Out of the blue" is an informal English language idiom that describes an event that occurs unexpectedly, without any warning or preparation. It is used as an adverb. "Out of the blue" is an informal English language idiom that describes an event that occurs unexpectedly, without any warning or preparation.[1] It is used as an adverb. The "blue" in the phrase refers to the sky, one from which a sudden plane falling is unexpected, thus it comes 'out of the blue'.[2] http://en.m.wikipedia.org/wiki/Out_of_the_blue_(idiom)