Most rust "busters" I'm familiar with are the phosphoic acid type, which is what causes the black oxide you've observed. (also the same thing in Coca-Cola that will "eat" a nail if it's left in a cup of Coke). Another old line product that has been used in body repair business for years is Rust Mort, again, contains phosphoric acid. Whether you grind it off or not depends on how much deposition there is. I've found it best to "grind" off as much rust scale as possible, abrasive grit dependent on how rusty the surface, or the black fiber wheels, or "flapper" discs (supposedly helps with cooling). Then use the chemical. That way, the deposition is minimal, mostly down in pits, and won't cause you to have to build an inordinately thick film of primer/surfacer to overcome it. Always rough up the metal surface before primer/sealer so that it has what's called "tooth", in order to help the sealer bond to the metal (even though most are epoxy, mechanical bond is essential).

There may be other types of chemistry besides phosphoric acid, and I get the feeling some of the "magic potion" companies are trying to impart some mysticism to their product for sales purposes, but it can't hurt to check around. A couple months ago one of the rod mags had an article on using muratic acid (probably because it's so easily obtained), and they were happy with the results. It left the surface with a greenish/grey look.