Over the years, I've come to the conclusion that hot blued parts do seem to hold color longer than rust blued parts. Especially on bits like, trigger guard tangs, forend irons, top levers, safety buttons and floor plates. Using fresh salts and good metal prep are course key in the quality of the finished product.
If rust blued parts are correctly prepped and rust blued in a "boiling hot water tank process" instead of the "old cold sweat box process" .........then, because of the heat penetration of the rust oxide during the boiling process, i.e. open steel pores and deeper penetration, and the numerous multiple coats and boilings, it has been my experience that a good rust blueing will out last and out wear the hot salt tank blueing process in most cases.......
Key word in all of this, is the word HOT..........
And yes, blood is the best blueing remover there is.....it is also great for pitting and ruining firearms....... always very interesting watching folks lay dead critters on their prized guns for the 'ol Kodak moment.......