clear blue water

blood in the water

In a competitive situation, the exhibition of apparent weakness or vulnerability by one party, especially when this leads to a feeling of vulnerability or greater pressure to perform on the part of the weak party, or enhanced expectation of victory by the others.

blood is thicker than water

Family relations and loyalties are stronger than relationships with people who are not family members.

blow out of the water

To trounce; to defeat thoroughly, as at a game or in battle.

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.