once in a blue moon

all at once

Unexpectedly; without warning; all of a sudden.

ask for the moon

To claim or desire something that one cannot have.

blue chamber

A forbidden room.

blue devils

Low spirits; depression.

blue moon

A long time.

blue note

The flatted 5th scale degree in the blues scale.

blue ribbon

Highly superior in quality, style, substance, prestige, etc.

blue state

A state of the United States voting Democratic in a given election, or tending to vote Democratic in general.