Hi, I was being facetious about the Sharpie, because I don't know what would work well.
Maybe a Deco-Color paint marker would work well. I don't know.
And yes, it's a matter of opinion. YMMV, as I said.
As for sticky labels, I have seen them on my share of old prints, as well as my share of stamps. (I used to work for an estate sales company, and dug through stuff from the turn of the century through the '80's; the average house we worked on was mostly full of stuff from the '60's and '70's.) While I have personally only put them on the back of mount boards, I think they are invisible on the back of prints as long as you don't shine light through the print. I also think the adhesive matters. You could probably do a little research with some label companies and see what they have to offer in terms of permanent archival adhesives.