Depends on how acidic the iron nitrate is, but probably a good idea to neutralize it. Wipe the stock down with a cloth dampened with household ammonia, will do the trick.
You can certainly use bone black over the stain. I wouldn't use just bone black, however. I think it would work better for you by mixing the bone black with whatever you're going to finish the stock with. By applying the mix in recessed areas and around carving, you can get some very nice depth, and variations of color. Take a look at some of Jim Kibler's pieces, he's a master at it.
Ed