I have made this before, and used it as a leather dye. I happen to machine cast iron, so that is what I use, as the old makers did. I have never tried steel wool, and I have only made one, one pint batch of the stuff, since a little goes a long ways. I have used it to stain leather only, so no idea how it might turn out on a stock. But my hunting pouch, 3 hip holsters and a shoulder rig have used less than a quarter pint of it, since it is strong stuff.
You really need cast iron for this, not steel or steel wool. I suspect this is why some are getting a bluish tint. My leather I have stained with this is not exactly black, but instead a very, very dark brown. The blue tinge may be coming from chromium in steel... Just a guess. Unless your cut nails are 100+ years old, they are probably steel. A good source for cast iron is old engine blocks. I would show a pick of my hunting pouch, but discovered my battery is dead... lol.
M. E. Pering