Originally Posted by
Winger Ed.
It's some chemical action going on in the dirt that does that. Acidic, alkali--- something.
But its a stain like gun bluing chemicals do to steel. It's not corrosion.
Years ago, I had a source for range brass that sold me the ugly black ones for about 1/3 the price of shiny stuff.
I was getting once fired .45ACPs for $20. per thousand--- shipped.
I'm a dry media guy and the black stains never came off.
It doesn't hurt the brass any more than a tattoo hurts your skin or ink stains 'ruin' your clothes.
As far as tarnished, I had been reloading for a few years before I got a tumbler.
I'd wash the brass in soap & water, dry, size, de-prime, etc. then rinse in Lacquer thinner before loading.