Brutalist ring question.

I have this sterling ring I’m cleaning up to sell and it has black something on the inside that won’t come off. It’s not tarnish but more like enamel? I am wondering if it was added intentionally to add depth to the piece? Is that something that is done?No marks so assuming artisan made. Is it @Hollyblue who is a silversmith? Anyway, all opinions appreciated.

 

It's a liver of sulfur patina,usually applied to low area to give depth and contrast on silver and gold at times.

 

Thank you Hollyblue!