Just to be clear, are people blowing down the barrel before or after loading.
Before loading, right after the shot
One can’t blow down a loaded gun
Obviously that’s beyond unsafe. But on a practical side, a patch and ball make a hard seal in the bore. There is blow by when shot, but that’s under thousands of pounds per square inch of pressure. Wads in a smooth bore won’t be as tight as a patched ball. However even that is a gas tight blockage.
One could blow with all ones might.
A trumpet player could blow with all his might
The angel Gabriel himself could blow and get no where near the pressure needed to blow down a loaded barrel.
Now if you were stupid enough to put a loaded gun in your mouth and blow the loaded gun, if it had a slow hang fire might go off. It would not go off by fanning the ember.
Just for an experiment after you have shot a range day, and you have cleaned your gun. A qt or more of water, dry patches, and oiled patch, your gun is an empty tube.
Now patch your jag, with an oiled patch, run clean to the breach, make sure there is nothing in the gun, withdraw the jag till you can just see it. Unscrew and leave in the bore, and now blow. You won’t move any air, you haven’t the king capacity to blow past even the relatively loose fit of you jag.
How could you blow past a tight PRB?