As long as it has not been previously shot and left loaded your fine. It's the salt from combustion of black powder that causes the rust from the fouling because it is hygroscopic.Black powder, even 4F is not very hygroscopic ,which I found out about a year ago after running some tests of my own in my bluing cabinet.
Elmer Kieth used to say he kept some of his black powder revolvers loaded for long periods of time without any rust.
One more thing should be mentioned and that is loading a clean revolver without making sure all oil is removed from the cylinder and nipples.I've gotten away with shooting out of oiled chambers if shot right away although the fouling was worse from the burned oil but I wouldn't want to load them with oil on the steel if going to be kept that way for long periods of time before shooting.
Acetone or alcohol work good for this.MD