I guess I'll pitch in my two cents here. I've never shot factory black powder cartridges, so I don't know how dirty the factory loads are, but my loads look just like any other shiny loads. They do smell more, though, because of the soft natural lube used with black powder. If there's actual grit or lube on the cases, I'd wipe them off before firing. Otherwise, I'd just shoot them.
Real black powder does not instantly rust your guns. Lots of guys that shoot black regularly wait up to a week before they get around to cleaning, with no ill effects. Most of the corrosion and pitting seen in older black powder firearms was caused by the corrosive primers that were in use at that time, and that's no longer a problem. Black powder fouling does draw moisture out of the air, so it will eventually rust uncleaned firearms.
It also turns your spent brass green, unless you clean it in warm water with a bit of soap. Just shake your fired cases in a jug half full of soapy water, and leave them in there over night. The next day, dump them out and rinse them off. Sometimes I brush out the insides, but you don't really need to if they've soaked for a bit. Dry them in a 200 degree oven for 15 or 20 minutes, dump them into your tumbler for a while, and your brass will be all shiny and clean with no green. My blackpowder reloads look like brand new sparkling factory ammo (which makes it easier for brass pickers to find my cases and get all my brass back to me, so it's worth a little effort).
Now, for cleaning the guns. I like warm water with a little Dawn in it. I swab out the barrel, wash up the cylinder, and wipe the exposed surfaces of my revolvers with a damp rag. They'll clean up pretty easy, especially if you were careful to remove ALL previous fouling from smokeless loads before you started shooting black powder. After the soapy water, wipe everything with damp patches with a little clean water, then dry with clean dry patches. If you miss a little fouling, or there's some inside the action, don't worry. A good dose of oil will prevent any rust. The oil prevents the fouling from drawing water out of the air and renders it pretty much harmless. I only detail strip my blackpowder guns once or twice a year, and none of them have rusted at all. I oil with Breakfree CLP, and it's been working very well for me and several other black powder shooters. Even Ballistol contains petroleum based products in it, so don't be too scared of using a little CLP to keep the rust off, especially if it's easier to find in your neck of the woods. Clean up is a snap, even when the gun was left oily with CLP right up until firing.
Blackpowder subs are not entirely non-rusting for guns, although most don't leave nearly so much fouling. Some are harder on brass than real black, and most draw moisture from the air, leading to rust, just like blackpowder. Pyrodex seems to cause the most serious trouble with rusting of any propellent I've ever used, so I don't use it at all anymore, and haven't for years.
Yesterday, I shot a total of 205 black powder handloads through 4 different firearms, and that's a pretty typical weekend for me. I also shot about 100 rounds in practice on Friday, from 3 different firearms. I cleaned all the guns and got them match ready today, and it wasn't any big deal. You sure can't wipe the gray-black smokeless powder fouling off of a cylinder face with just a quick swipe with a damp rag, but a black powder fired gun will shine up like new.
