blue moon

ask for the moon

To claim or desire something that one cannot have.

blue chamber

A forbidden room.

blue devils

Low spirits; depression.

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.

blue wall of silence

Strict secretiveness maintained by the members of a police force with respect to information which might be contrary to their interests, especially information concerning questionable police actions.

bolt from the blue

A complete surprise; something totally unexpected.