Do it like this:
Neutralize whatever cold blue you are using right now by washing the piece off with hot water and detergent. *Note* Cold blue is toxic. Dispose of waste water accordingly and do not use containers for anything else. Rinse it well to get rid of any detergent residue.
Restore the finish to what you want.
Thoroughly clean the piece with acetone.
Put on your gloves. I use Ansell TNT nitrile gloves, but any non-powdered, non-reactive glove that will keep your fingerprints off the metal will do. You will NOT now touch the piece with anything that may carry oils or contaminants until you are finished.
Thoroughly clean the piece with alcohol. Rubbing alcohol or methyl alcohol (gasline antifreeze) will do. Lay the piece on a clean rag to dry.
When dry, you may begin bluing. Instructions will differ depending on which blue you are using, but in general, shake the contents of the bottle well, and apply to the metal using a saturated patch. Cover the entire surface you are bluing quickly and as evenly as possible. It's kind of like paint. If you let one area dry and then paint up to it, it will leave streaks. If you overpaint a previously coated area, it will leave streaks.
Let the blue do it's work for a bit. After the first coat is dry, I usually overapply a second, or even third and fourth coat, until I have a thick application built up. This will generally look like ####. Depending on the type of blue you are using it should now look like a creamy rust is all over the surface.
When that is dry, but not too dry, rub off the excess oxide with fine steel wool. An even tone of blue should be revealed. Repeat the coating and rubbing with steel wool until you have the desired depth of color. This may take many applications (40 on one knife that I blacked the blade on).
When the desired finish is achieved, again neutralize the blue by washing in hot water and soap.
Again clean the piece with alcohol.
Oil the hell out of it.
And you are done.
