I've heard that a lot of iron gall inks have another (non-waterproof) dye mixed in to darken them. The iron gall component should be pretty permanent once it sets, but that can take a while (possibly a week or more) depending on the paper you use it on, and I don't think it's ever really black. Of course, this is all hearsay, mostly from FPN; I've never used one of these myself.
I've heard that a lot of iron gall inks have another (non-waterproof) dye mixed in to darken them. The iron gall component should be pretty permanent once it sets, but that can take a while (possibly a week or more) depending on the paper you use it on, and I don't think it's ever really black. Of course, this is all hearsay, mostly from FPN; I've never used one of these myself.