Which ever brand you decide on, try this tip: Degrease the parts to be blued and make sure that you don't touch them with bare fingers, heat them in boiling water for a few minutes before applying the "cold" blue. I've use this with the Outers paste cold blue, resulting in a deep blue/black finish that has not started to wear, yet.
Apply cold blue to hot part, let stand, rinse, re-heat and re-apply until the color you want is reached. Rinse well and then apply a coating of good gun oil and let it set