Open English Wordnet
Adjectives
(s)unanticipated, unforeseen, unseen, unlooked-for, out of the bluenot 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”, “unseen problems”
Adverbs
(r)unexpectedly, out of the bluein a way that was not expected“her brother showed up at the wedding out of the blue”