- Out of the blue: unexpected
- True blue: to be loyal or faithful
- Once in a blue moon: an event that occurs infrequently
- Blue ribbon: first place; to describe something as being of the highest quality
- Blue blood: an aristocrat
- Blue law: laws about morality issues
- Blue comedy: jokes about socially taboo subjects
- Blueprint: a detailed design of an object or idea
- Blue plate special: a special priced meal at a restaurant
- Bluestocking: a woman with strong scholarly interests
- Feeling blue: to feel sad or unhappy
