I've liked Brownells Oxpho-blue for small parts. Warm the metal before applying, then oil them afterward. Gives a good, blackish finish with just one application. Not sure for larger parts, like your barrel, as the Oxpho can be hard to get a uniform color, esp. for something large like a barrel. Did you rub-in multiple coats?
And you burnished off the rust with mere steel wool? No sandpaper?
I've also tried Birchwood Casey Bluing paste, and Brownells 44/40 liquid bluing. Neither of these worked as well for me as the Oxpho paste, but I'm interested in how you got such a uniform color with the BC blue paste.