out of a clear 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 come out of the clear blue sky.
out of a clear (blue) skyas a complete surprise.
1992 New Yorker The latest revelations… about the marriage of the Prince and Princess of Wales may have induced disbelief, but they did not come out of a clear blue sky.