Looks to me like a random aspect of the toning, perhaps. Maybe a light hit there which disturbed the toning a bit. Unless it's raised, in which case I don't know.
I use baking soda to shine up all my metal detected finds.. Some people act like I murdered someone when I mention that The silver coins have been buried and already scratched for decades!
when I have time, if I remember, I'll tell you how to clean those silver coins that you have from the beach, and have turned black.
Okay, here goes: Black silver coin, unknown as to whether there is any design left. 1) get aluminum foil; 2) place coin next to it; 3) [Not see the note to follow} I forget which way, but I believe you put the coin on the shiny side of the foil; 4) put a large drop of water on each side of the coin, or do one side at a time; have old cloth rag handy; 5) fold over the aluminum foil on the coin, cover completely, and tight. 6) You'll know very quickly if it is working, you'll see the foil start to bubble after a few [probably no more than 5 or 6 seconds]. Let it sit If it isn't bubbling, flip over the foil and use the other side. DO NOT TOUCH! It is very hot. When it is still very warm, but not hot feel the foil off the coin, it may stick in parts. Put coin on cloth, rub hard, and you should see the black come off. Flip over & repeat. I have done this with really black coins, from salt water. I found a Barber dime that way, sure it had no numismatic value, common date, but I had the fun of seeing the coin. Sometimes you can see the date, sometimes it is hopeless. but you should be able to see the design. I do not advise this for any coni with numismatic value, proceed at your own risk. Not for kids -- the coin will be very HOT! [I hate posting warnings, but you all have been warned.] You can try with a junk Roosie first, for practice, and then do the black ones. You can repeat the above procedure if necessary.
@Mojavedave, can you determine if it is raised or not? From just the photo, it appears to just be a toning difference that gives the appearance of extra metal.