I'd guess sulfur. The blackest coins I've owned were
Peace dollars and walkers close to mint state. Maybe due to the enormous amount of low quality coal burned for heat, power and transportation ca 1910-1930. If you choke down the air when you fire high sulfur fuels the sulfur is emitted as H2S.
Here's how to do it safely at home. Unreduced yellow native sulfur or K2S are very effective.
http://www.metaldetectingworld.com/...ishing.shtml