Princeton's WordNet

1. unanticipated, unforeseen, unlooked-for, out of the blue(p)(adverb)

   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"

2. unexpectedly, out of the blue(adverb)

   in a way that was not expected

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

Wiktionary

1. out of the blue(Adverb)

   unexpectedly; without warning or preparation

• uit die blouteAfrikaans
• فَجْأه، على غَيْرِ انتِظارArabic
• изведнъжBulgarian
• de surpresaPortuguese (BR)
• zčista jasna, znenadáníCzech
• aus heiterem HimmelGerman
• uden varselDanish
• απροειδοποίητα, ξαφνικάGreek
• como caído del cielo, de improvisoSpanish
• nagu välk selgest taevastEstonian
• ناگهان؛ غیرمنتظرهFarsi
• äkkiarvaamattaFinnish
• à l'improvisteFrench
• פִּתְאוֹםHebrew
• बिना कोई खोज-खबर दिएHindi
• iznenadaCroatian
• hirtelenHungarian
• tiba-tibaIndonesian
• úr heiðskíru lofti, öllum að óvörumIcelandic
• all'improvvisoItalian
• 出し抜けにJapanese
• 불시에Korean
• kaip perkūnas iš giedro dangausLithuanian
• pēkšņi; bez brīdinājumaLatvian
• tiba-tiba datangMalay
• onverwachtDutch
• uten varselNorwegian
• niespodziewanie, ni stąd ni zowądPolish
• ناگهان؛ غیرمنتظرهPersian
• نا څاپه ، غیرمتراقبهPashto
• de surpresaPortuguese
• pe neaşteptateRomanian
• внезапно, без предупрежденияRussian
• znenazdaniaSlovak
• kar na lepemSlovenian
• iznenadaSerbian
• utan förvarning, helt oväntatSwedish
• ไม่ได้บอกล่วงหน้าThai
• ansızın, beklenmedik bir zamanda, pat diyeTurkish
• 突然地Chinese (Trad.)
• несподівано; як грім з небаUkrainian
• اچانک ، دفعۃًUrdu
• hoàn toàn bất ngờVietnamese
• 突然地Chinese (Simp.)