A few years ago I picked up a product from Caswell Plating that appeared to give a true black color but have yet to try it. They now have few items which may work. Check out their website: https://www.caswellplating.com and look at the sections on "metal antiquing" and "everything for firearms".
I may have to take another trip there with a bare frame and see what they recommend. They are less than an hour from me.
Pete