out_of_the_blue
1. [ adjective ] not anticipated
Synonyms: unforeseen unlooked-for unanticipated
Examples:

"unanticipated and disconcerting lines of development"- H.W.Glidden "unforeseen circumstances" "a virtue unlooked-for in people so full of energy" "like a bolt out of the blue"

Related terms: unexpected
2. [ adverb ] in a way that was not expected
Synonyms: unexpectedly
Examples:

"her brother showed up at the wedding out of the blue"

Related terms: unexpected unexpected
Similar spelling:   out_of_thin_air
  out_of_danger