The word blue has several meanings:

1. A color like the sky on a clear day or deep ocean water. Blue sits between green and purple in the rainbow. Different shades range from pale baby blue to dark navy blue. Many things in nature are blue: bluebirds, bluebells, blue jays, and of course the sky and sea.
2. Feeling sad or melancholy. When someone says they're feeling blue, they mean they're downhearted or gloomy. This usage is closely linked with the slow, mournful style of music called the blues, which expresses feelings of sadness or hardship. If you're disappointed about missing a friend's party or worried about a test, you might feel a little blue for a while.
3. Something happening suddenly and without warning is said to come out of the blue, like an unexpected phone call from an old friend or a surprise quiz. The phrase suggests something appearing from a clear, empty sky where you'd never expect anything to emerge.

Blue is one of the three primary colors in art, meaning you can't create it by mixing other colors together. But you can mix blue with other colors to create new ones: blue and yellow make green, blue and red make purple.