Something that is blue is the colour of the sky on a sunny day.

The blues is a type of music which was developed by African American musicians in the southern United States. It is characterized by a slow tempo and a strong rhythm.

If you have got the blues, you feel sad and depressed.

If you are feeling blue, you are feeling sad or depressed, often when there is no particular reason.

A Cambridge blue or an Oxford blue is a man or woman who has played for Cambridge or Oxford University in a particular sport.

Blue films, stories, or jokes are about sex.

If something happens out of the blue, it happens unexpectedly.