Superb find! I'm sure the blackness is due to water in the soil or having been dug near a body of water in a low area. That's usually what does it for me. SLQs are my favorite coins and that one has amazing detail!
I know I will get mixed signals but I decide to lightly clean this coin. I figured I will never sell any of my finds and I wouldn't even know how to get a coin graded so I cleaned it up a bit so I had something a little more appealing to look at. It still has some discoloring but I didn't want to scrub it at all. Here is the outcome.