I use Laurel Mountain Forge Brown/Blue. This is a rust blue that will give you a dark deep color in 4-6 applications. Easy to use, but you need to make sure the barrel(s) are prepared correctly. Laurel Mountain Forge
They make a super blue kit that is almost black.I am doing a barrel right now.I will post how it turns out.I will say it takes a few coats and time in between coats.I'm using the regular kit though.
It will be dark.I've done it before.
I used B/C Super Blue to do the metal parts on the smoothbore flintlock kit I built last year. I wanted something dark because I planned to hunt turkeys with it. Man did that stuff work. They shoulda called it Super Black.....Its real close to black in color.
We used regular B/C bluing to do my son's barrel in his Traditions flintlock kit. It turned out much better than I had thought it would.
BTW - Don't use Laurel Mountain Forge browning/rust bluing solution. That stuff is garbage. I followed the directions to the letter and it came out looking like dog crap!