Actually,normal exhaled breath only contains about 4.5% carbon dioxide and about 21% oxygen, which is enought to fan an ember into flame. Anyone who has ever breathed on a fire to help it get going has seen first hand that there is plenty of oxygen in exhaled breath. This is why is is not advisable to blow down a barrel after firing. The smoke in the barrel has created an oxygen depleted environment that will not support a fire from a glowing ember but blowing down it could cause it to flare up. I think the chance of this happening is pretty small with todays blackpowder but why take the chance.

I have always been told that the reason for blowing down the bore was to soften any fouling from blackpowder. Exhaled breath has a relative humidity of 100% so the moisture from it would loosen fouling, making a subsequent reloading easier to do, without swabbing. This might have been pretty beneficial to frontiersmen, whose lives might have depended on a quick second shot, but isn't too important today. Modern lubes can keep blackpowder fouling soft and make reloading consecutive shots, with a patched ball,fairly easy to do without swabbing between shots.