As a language expert with a focus on communication and expression, I can provide insights into the idiomatic use of the term "blue" in English.

In English, when someone says a person is "feeling blue," it's an idiomatic expression that means they are sad or depressed. This usage doesn't refer to the color blue but rather to an emotional state. The phrase likely originated from the association of the color blue with melancholy or sadness in various cultural contexts.

Here's the answer with the key term in red:

Feeling blue is an idiom that describes a person's emotional state when they are sad or depressed.

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