Farmer's Reducer, in about a 1/10 dilution and used VERY carefully, will
remove the fog without affecting too much of the other densities. In case
you've forgotten Farmer's Reducer (named for a Mr. Farmer, and not an
agricultural designation) is plain hypo with potassium ferricyanide. Photo
stores used to sell packets of the stuff, but I haven't seen it lately. It
would theoretically be possible to use a color processing bleach-fix, but
many of the formulas I know of won't work in a very dilute solution. And
you do need a very dilute solution as otherwise the stuff works fast - too
fast to protect the image. Frankly, before using a reducer on an
irreplaceable image, I'd make a copy. And if you're going to make a copy,
you can try to compensate for the base fog level!
If you can get the two chemicals, but can't find the formula, email me and
I'll provide it. But it's in just about every darkroom book published prior
Old-timers used it a lot to brighten up silver prints.
Sil Horwitz, FPSA
Technical Editor, PSA Journal