Idioms: 'out of the blue' or 'out of the blues'?

Hello there everybody,
Is is ‘out of the blue’ or ‘out of the blues’ ?
I hope someone could help me!
Thank you.

I think that it would be definitely “out of blue” or so does my dict PWN explains this idiom. e.g. to say sth out of the blue.

:slight_smile: