[cmcminn]

I have experimented a little with the BO kit on carbon steel putter (golf) heads and encountered the same problem you have with the cold process. What I found that works is to use hot water (app 120*F) when mixing the blacking solution. I then preheat the putter head to app 200*F using a heat gun (paint stripper) and then quench it in the blacking solution. 5-10 seconds gives a dark black uniform finish on a putter head. I then use the heat gun to dry the part (never wiping) and then quench again in the sealer (room temp) using the heat gun to again dry the part . Sit aside until it is cool to the touch. I then wipe any residue off and after the initial wipe it will usually wipe clean from that point on.
Some other observations - the black oxide has a terrible odor that never seems to go away and also my putter heads will eventually begin to oxidize(rust) meaning I have to repeat the process every few months.