Out of the blue: In a way that was not expected. （突然地）1. From an unexpected or unforeseen source: criticism that came out of the blue; 2. At a completely unexpected time: a long-unseen friend who appeared out of the blue.
中文释义：out of the blue这个短语来自于a bolt out of the blue，相当于中文里的“晴天霹雳”，用来表示“突然地；出乎意料地”。所以blue在这里最初代表的是蓝天。例句：Mike has decided to quit his job out of the blue, and go travelling for a year!（Mike突然决定辞职，然后花一年时间去旅行！）；英国女歌手阿黛尔(Adele)的《Some one Like You》中有句经典歌词 I hate to turn up out of the blue uninvited.表达的意思就是“我也不想这么突然地不请自来。”