Both throwing the ramrod down a bit and blowing through an EMPTY barrel had reasons to do so.
On the blowing the reasons are to have your humid breath make the powder fouling softer, to see a puff of smoke coming out of the touchhole to be sure it was clear, and to burn out any spark left behind. A blast of air will do that. A low oxygen, just fired barrel doesn't have much air. Hence, lingering sparks. Cannon barrels were swabbed after firing to extinguish any lingering sparks or embers from the gunpowder, which could ignite the next powder charge and endanger the loading crew.
I can't imagine why anyone would try to blow down a LOADED barrel, that's an entirely different story. And extremely dumb to ever put any part of your body over the bore of a loaded gun of any type. Ever.
"How do you know it's not loaded?" Well, you just FIRED the gun, and saw the bullet strike. Pretty certainly it's not loaded after a big boom, and a bullet flying out. "Could someone mess up and mistakenly blow if it's their habit?" Yes, if people make mistakes an ANY way loading and shooting black powder, accidents happen. I prioritize the risky things. For example, I don't use the palm of my hand on the end of the ramrod to seat the bullet. IF a lingering ember sets off the charge at that point, you've got a ramrod and a ball shooting through your hand. I'd say there are 3 ways to prevent sparks - blow down the bore, wipe with a wet patch, or wait a few minutes between loading. I'm not advocating anyone blow down the barrel mind you....and not saying if I do.