I once had to frame an official portrait of our State Governor. It was a beautiful black and white photo printed on heavy paper with the mattest (?) finish I have ever seen and, of course, his uniform was predominantly black It had been rolled for posting and the edge of the paper has slightly scuffed the surface at one point.
I pointed it out to the client straight away and with his go-ahead used a cotton bud dipped in rottenstone powder to lightly abrade the scuff and it disappeared like magic.
Disclaimer: The photo could easily have been replaced otherwise I would not have touched it.