What is black shampoo?

Could you possibly elaborate on the concept of "black shampoo" for me? I'm quite intrigued by this term but I'm not entirely sure what it refers to. Is it a specific type of shampoo with unique properties? Or perhaps a colloquial expression that I'm not familiar with? Could you explain its purported benefits, if any, and how it differs from traditional shampoos? Additionally, is it widely available or is it a niche product? I'm eager to learn more about this intriguing topic.