Think about the chemistry of the process.   You want to get some red oxide, done by putting on an acid solution to rust it to red Fe2O3. Some bluing acids will do this directly, others want a high-humidity, but not dripping, environment.  Then boil it in water where the boiling has driven any dissolved oxygen out of the water.   The Fe203, in the presence of iron and absence of oxygen, forms black oxide , Fe304, in micro-pits on the surface.  Brush off any loose oxides and repeat until you get a glossy or matte blue-black, depending on the initial polish and subsequent corrosion.   Since the color is in microscopic pits in the surface, it is very durable, unlike the thin layer of black in "nitre jobs".

CHRIS

Are you boiling it and carding it after rusting it?

I follow the procedure in "Firearm Bluing and Browning" by R.H Angier p 80, "zischang Browne" as well as using Brownells Dicropan IM.

I polish the metal, apply the rust solution and allow it to rust over nite or for a couple of days as needed to develop a rust brown/copper surface. Then boil it in distilled water, let it dry and card it with a 0.007 dia wire wheel. Repeat 5 to 7 times for a good finish.