out of the blue

adjectives

not anticipated
"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"
out of the blue, unanticipated, unforeseen, unlooked-for

adverbs

in a way that was not expected
"her brother showed up at the wedding out of the blue"
out of the blue, unexpectedly