Is black and white an idiom?

"Black and white" is not an idiom in the traditional sense. It's more commonly used as:

* A descriptive phrase: To describe something that is literally black and white, like a photograph or a zebra.

* A metaphorical phrase: To describe something as clear-cut and unambiguous, with no shades of gray.

While it's not a traditional idiom, it's a common expression with a clear metaphorical meaning.