I think it might just be the rubber wearing out,
The worn rubber dseteriorates, and becomes small rubber particles that have dried up and become solid (ie not rubbery anymore, from the heat/sun etc)
combine this with the water and anytime you go near the rubber with a chamios or whatever you smudge this black filth everywhere. I had this from when I was wet sanding, there for I believe it's worn rubber.
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