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Adjective: out of the blue
  1. Not anticipated
    "like a bolt out of the blue";
    - unanticipated, unforeseen, unseen, unlooked-for
Adverb: out of the blue
  1. In a way that was not expected
    "her brother showed up at the wedding out of the blue";
    - unexpectedly

See also: unexpected

Encyclopedia: Out of the blue