While running coax cable in our newly-purchased house, I removed a receptacle and box (which was not attached to or supported by anything but paint and drywall mud, but that's a story for another day). The cable coming in was wrapped around some insulation and the portions of it touching the insulation were covered with this black residue.
I have read elsewhere that this can be caused by tar from old insulation, but the insulation in this case does not appear to be particularly old, and the insulation itself doesn't seem to have any discoloration.
The residue appears to only be on the surface, as you can see from the couple places where I scratched it off with my fingernail to expose the cable sheath underneath. The sheath itself does not appear to be compromised. (The silvery patch on the cable in this picture, which I had initially thought was a hole of some sort, appears to have just been a piece of foil or other material as I was able to peel it off and reveal the white sheath beneath.)
Is this cause for concern? Or is it in fact tar from the insulation, or something else entirely?