black market: The black market refers to the illegal buying and selling of goods or currencies.
Be careful of what you buy on the black market - it's not always good quality.

black mood: To be in a black mood means to be irritable, angry or even depressed.
You'd better keep away from Bill today. He's in a black mood.

black out: If you black out, you lose consciousness.
When Tony saw the needle, he blacked out.

black sheep: The black sheep is one who behaves very differently or badly, and is considered disreputable by the other members of the family.
Joe was the black sheep of the family, always getting into trouble.

black tie event: This expression refers to a formal event at which men are required to wear a dinner jacket, or tuxedo, and a black bow tie.
I need to know if it's going to be a casual get-together or a black tie event.

in the black: To say that a person or organization is in the black means that they are financially sound, have a positive balance on their account and that they owe no money.
beaten black and blue: If a person is covered with bruise marks caused by being hit, they have been beaten black and blue.
The passenger was beaten black and blue by a gang of thugs.
black and white To say that something is in black and white means that there is written proof of it.
It's an obligation. It's in black and white in your contract.
blue around the gills If a person looks blue around gills, (or green or pale) they look unwell or sick.
You should sit down. You look a bit blue around the gills.
blue chip company This term refers to a company with a solid reputation for the quality of its products and the stability of its growth and earnings.
It's usually safe to invest in a blue chip company.
blue in the face If you do something until you are blue in the face, you try unsuccessfully to do something for a very long time.
I explained the situation until I was blue in the face but she wouldn't change her mind
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feel blue: To feel blue means to have feelings of deep sadness or depression.
I'm going to see my grandmother. She's feeling a bit blue at the moment.

blue-eyed boy: A blue-eyed boy is someone's favourite.
He's the director's blue-eyed boy!

once in a blue moon: If something occurs once in a blue moon, it happens very rarely.
Bill has very little contact with his brother. They see each other once in a blue moon.

out of the blue: If something happens out of the blue, it happens very unexpectedly.
I had nearly given up hope when out of the blue I was offered a job.

(scream) blue murder: Someone who screams blue murder shouts or complains very loudly as if something very serious has happened.
The crowd started screaming blue murder when the football match was interrupted.