I like to use gallic acid to create iron gall ink using the new cyanotype process. I think this is the closest to a black silver print possible with this process. It is my experience that for the redevelopment (after bleach) to use distilled water and freshly mixed gallic acid. The mixture should be used immediately as once the gallic acid oxidizes it will cause the pigment to migrate throughout the paper causing spots, bleeding, etc. If it is fresh, quickly done, and well washed, it will work. If you dally, you will have a problem.
I have done extensive research into the stability of the iron gall ink. Will it last 1000 years? maybe, depending on how it is stored. Do I care? not really. It only needs to last me 75 yrs until I don't care anymore.