Idioms are phrases with figurative meanings that are different from the literal meanings of the individual words. In this case, "out of the blue" means something unexpected or sudden, like a surprise or a shock.
Here's why it's not a rhetorical device:
* Rhetorical devices are specific techniques used in writing and speech to create emphasis, clarity, or emotion. They are usually intentional and deliberate.
* Idioms are common expressions used in everyday language. They are often used without conscious thought about their meaning.
However, "out of the blue" can be used in a rhetorical way. For example, a writer might use the phrase to create a sense of surprise or suspense in a story. In this case, the idiom would be functioning as a rhetorical device to achieve a specific effect.
So, while "out of the blue" itself isn't a rhetorical device, it can be used rhetorically to enhance writing or speech.