While I have never witnessed it myself,,, I have read reports by other shooters suggesting that under certain unusual circumstances unburnt powder can build up on the breechplug and hold a spark. They cite instances of guns firing during the process of seating a ball on the powder charge resulting in ramrod being shot up into the air or through the palm of the loaders hand . Since the guns had hammer down and frizzen open they feel a spark held in unburnt powder is the only possible explanation for the premature firing . So the concern is that blowing down the barrel might cause such a deposit holding a spark to ignite the unburnt powder and cause damage to the lips,face, and or lungs of person blowing down barrel. I have never witnessed such an accident nor do I personally know anyone who has actually witnessed this type of accident. I am only relating what I have read on other forums as food for thought.
I think maybe you are misunderstanding what you've "read on other forums", because what you are describing makes absolutely no sense.
If you drop a charge of powder down the barrel (followed by a PRB) and "fire" it,.... there is no way you can get that same charge of powder to "fire" twice no matter how much you blow down the barrel.
Now if you're saying that once a charge has been "fired" and you drop another charge of powder down the barrel THEN blow down the barrel, and that doing so might cause the second charge to go off,..... WHY would a person EVER blow down a barrel AFTER dropping a fresh charge of powder in??
The whole premise of blowin' down the barrel, is to soften the fouling from the previous shot, and to let you know the vent/nipple is open.
Some also claim that blowin' down the barrel will also help extinguish any remaining "embers" that
might remain in the breech after "firing". (
IS THIS WHAT YOU'RE TALKING ABOUT?)