I use Oxphoblue
I degrease the parts or the barrel and use a heat gun to get the part very hot. It blends pretty well with the hot blued guns that are the black color, but it won't match worth a flip for any of the slow rust blued guns. I use a cotton ball to apply it and I let it sit a little longer than a minute, but make sure to keep the part wet with the creme. Do several treatments to get the dark color your looking for. If the gun isn't very dark, it should only take 2 times of wetting the metal, more than 4 to get it to match a newer gun. I apply Breakfree afterwards and buff it good with 0000 steel wool. I then wipe it clean and reapply the breakfree and let it sit for a day or two before handling the gun. Good luck with it.