This has been and on and off issue for us. We first noticed these smear-like specks about a year ago. Replaced expansion tank and anode rod. Came back a few months later and brought along larger specks but these larger one's didn't smear.
We wound up replacing the PRV on the main line thinking it was the rubber from there (but honestly, it didn't even look that bad) and while we don't have the large specks any longer we still see the smear-like specks in the bath tub and dishwasher.
I think I will say that it's no where near what it used to be...but it's not resolved. We are starting to see the black specks also on hand rags that we would use in the shower and the tub (it's not mold, which I thought could have been the case...too many rags it's happening too) ODD...very ODD. Especially since we can't really see them until afterwards. We can't see it when we run the tub/shower. It sort of just shows up and is there ready to be smeared.
I had lowered the water heater some since it's summer time and within a week of doing that we saw an increase in the smear-like specks. Increased the heat and there's a decrease but still exists.
I'm not sure there's been any resolution for some folks dealing with this.
As for cleaning the buggers, the only thing I found that works is soft scrub but I have to scrub real hard to get it removed.