Once in my life, The moon is blue.

一生只有一次，月亮是蓝色的。

youdao

Once in a blue moon. It doesn't mean that the moon is blue.

（千载难逢），它的意思可不是月亮是蓝色的。

youdao

This is because some indirect sunlight still reaches the moon after passing through the Earth's atmosphere, which scatters blue light.

这是因为通过地球大气层呈现蓝色的太阳光仍旧会间接抵达月球。

youdao

The moon is full and the clouds scudding across its surface cast blue shadows on the snow.

BBC: Snowbound Slovenia