out of the blue:出乎意料;突然;晴天霹雳

His resignation came right out of the blue. 他的辞职来得很突然.

she had no idea that anything was wrong until he announced out of the blue that he wanted a divorce.
在他突然宣布要离婚之前,她还不知道出了什么问题.

once in a blue moon:very rarely 极少;难得;破天荒

Sue's daughter only visits her once in a blue moon.
苏的女儿很少来看她.