Blackpowder is mildly corrosive, depending on the brand and how much nitrates and sodium the residue contains. The residue from blackpowder is easily removed with water. It will attack the brass cases if left in a extemely humid condition for an extended period of time.
Pyrodex is treated with large quantities of potassium perchlorate, and that is what causes it to be so extremely corrosive, and has actually ruined some guns that did not get followup cleaning and oiled. That is why Hogdons themselves caution tht pyrodex is highly corrosive and guns need to be cleaned immediately. It as with all the so called blackpowder "substitutes" will ruin brass if not cleaned promptly. There are examples of brass cases shot with 777 actually desinigrating a year later...
The wives tales about blackpowder are numerous and widely abound. But in actual fact cleanup with blackpowder is extremely easy, can be accomplished with nothing more complicated that plain water, and a gentle coating of 3n1 oil afterwards.
Cleaning the cases is the biggest job, but is relatively painless, a simple shaking of a jar filled with simplegreen , water and the deprimed fired cases and then rinse with clean water, dry the cases and go. If you are going out in public and don't want to be shameful of dull cases, simply run them thru vibratory cleaner with your favored case cleaning media concoction.