What are the two black marks on the back of the pitcher’s mound- I’ve noticed them just recently at Fenway Park. I don’t know if they exist at other parks. They are only visible on TV from the center field camera. They look like two black pitching rubbers, about a third of the way up the back of the mound, one on either side. Usually the rosin bag ends up on the ground between them.
Not sure. But not a rosin bag- probably a hard rubber “shoe cleaner”, used by pitchers to get clay/mud out of cleats. This is opinion only. I have not seen one anywhere else.
I believe “shoe cleaner” is on the money. I’ve seen pitchers digging dirt out of their cleats with them, and I can’t imagine that they’re carrying those things out to the mound themselves.
I don’t know - these black things are pretty big, each is about the size of the pitcher’s rubber (24inches x 6inches). I’ve seen the little shoe cleaners they sometimes use that are the size of a table knife.
And why would they need TWO shoe cleaners on the mound? One for each shoe? 
Has anybody seen these on TV in a stadium other than Fenway? They are very visible from the center field camera, especially a longer shot, e.g., when a runner is on 2nd.
Those black things are gravemarkers. Buried underneath the one on the right is Ted Williams and the one on the left is occupied by the body of the last man who was alive when the Red Sox last won the World Series…
Let me explain. They are rubber mats, that have hard metal wires that one cleans the shoe on. They are not “portable” like tongue depressors that pitchers put in back pocket. They are like small front door mats. Probably needed now because of all the rain there.
I think Gaylord Perry stashed his sandpaper, nail file, vaseline, K-Y jelly and thumbtack under the mats.
Good answer TREE BOY , but I think that Williams hole is full of dry ice.
I suspect that the 2 black objects you refer to are the 2nd baseman and centre fielder, perhaps foreshortened by the 2 dimensional camera view?
They would be proven to be mats if Perry put his keys underneath.
Now that I’m looking for them, I still haven’t seen these mats on other pitcher’s mounds except Fenway Park. Maybe it’s the RedSox way of being courteous to the Yankees, Toronto , etc - “Welcome to Fenway Park - Walk All Over Us”.