R.B.
Haven't seen this myself, but if you're using a heat gun, you might be getting it too hot. Chromates do NOT like excessive high temperatures. Caswell Blue is maximum 150º.
I occasionally use a heat gun too, but I "calibrated" it.
Using a thermometer, I held the heat gun at different distances from the thermometer, until I found the right distance for 140º. It turned out to be 3-3.5 feet for my gun!
Sean