这个短语源自 “a bolt out of the blue (sky)”（晴天霹雳）。蓝色代表晴朗的天空，在毫无预兆的情况下突然出现某事，就像晴天里落下的雷电。

• I hadn’t heard from him in years, then he called me out of the blue last night. ^{我好几年没他消息了，结果他昨晚突然给我打了电话。}
• The offer for the new job came out of the blue. ^{这份新工作的邀约来得非常突然。}
• She decided to quit her job out of the blue, surprising everyone. ^{她突然决定辞职，让所有人都大吃一惊。}