I prefer either Novum Solutions "Blue Wonder" (have a black version too) or Brownell's Oxpho Blue (cream version works better than liquid version for me) to cold blue a firearm. I've had good and bad results with both products, but they have worked better overall than other cold blue products I've tried. Both products wear quite well and the Oxpho is easier to use and less costly than the Blue Wonder. If I'm redoing an old rifle not worth much, I'll use the Oxpho. Use the Blue Wonder on firearm I really am concerned about turning out nice.
A lot depends on the metal you are working with. Basically a barrel takes cold blue better than a receiver because a barrel is softer. Have a barrel on an old BSA rifle from the late 60s ready to be cold blued and expect it to turn out nice. Will not try to do the receiver on the rifle as it is in better condition than the barrel and as I mentioned already, its hit and miss on a receiver due to its metallurgy.