Some come out of the ground with more oxidation than others - depends on the soil, mineralization/fertilization, coin condition, etc, etc.
Run a little water on that Merc, then pour a helping of baking soda onto the coin, and rub with your fingers for a minute or two, and that oxidation will disappear fast. If that doesn’t work for some reason, you can hit it with a regular pencil eraser - literally rub the eraser on the coin like a crooked banker fixing his ledger and it’ll shine up in a jiffy!
From what I've seen, and noticed among others, is that silver coins previously buried for decades under concrete are sometimes so oxidized they're black. Not all, but some.
On the other hand, under normal undeveloped soil conditions, I've never had a high percent silver coin come out anything other than shiny.