Black represents water. In China, as elsewhere in the world, black 黑 symbolizes something serious, very formal. It was the color wore by the imperial dignitaries, much like the outfits of our lawyers back in the West. Black also expresses the secret in Chinese, something that is happening in the shadow like the mafia.
https://www.windchimeschinese.com/windchimes-blog/2018/5/29/color-in-chinese-culture