Most rust browning will turn very dark or even black if you just boil the parts in water for 5 minutes when the rusting is finished
If you are going for the rust blue intentionally (black color, even though it's called "blued") try the Brownells product. It works very well. The Pilkington product is also excellent, though it's very expensive for what you get. most folks think it's over priced, and that's why Brownells brought theirs out in the first place