The problem is the black stuff is dirt, rubber particles and other debris that is stuck to the adhesive on the underside of the mylar.
Yes, you can get in there and remove it but in the process you're also removing some of the adhesive from the mylar at the same time.
It will make it look better temporarily, but will eventually come back and come back further underneath the mylar where there is more adhesive since in the cleaning process you removed the adhesive nearest the edge.