out of a clear blue sky

out of a/the clear (blue) sky

Completely unexpectedly; out of nowhere. She seemed so surprised by the news that it must have come out of a clear blue sky. You need to work hard—opportunities don't just appear out of the clear sky.
See also: clear, of, out, sky
Farlex Dictionary of Idioms.

out of a clear blue sky

Also, out of the blue. Without warning, suddenly, as in Her offer to help us with the fundraising came out of a clear blue sky, or We got a check from Aunt Ruby out of the blue. These metaphoric terms allude to something dropping unexpectedly from the sky. [Late 1800s] Also see out of nowhere.
See also: blue, clear, of, out, sky
The American Heritage® Dictionary of Idioms by Christine Ammer.

out of a clear blue sky

If something happens out of a clear blue sky, it happens completely unexpectedly, with no warning. The collapse of the airline did not come out of a clear blue sky. Note: People sometimes use this expression leaving out either clear or blue. Unlike many previous sharp falls in the stock markets, this one cannot be said to have come out of a clear sky. Note: This expression compares an unexpected event to a bolt of lightning from a blue sky. The expressions `out of the blue' and `a bolt from the blue' are based on a similar idea.
See also: blue, clear, of, out, sky
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • come out of a clear blue sky
  • come out of a/the clear blue sky
  • come out of the blue
  • be hit for six
  • come from nowhere
  • appear out of thin air
  • appear, etc. out of thin air
  • as it turned out
  • as it/things turned out
  • blank out