I used molasses on some of my Chevy parts when we were still at the old house. It seemed to be working pretty well at first so I got ambitious and setup a spray shower to start doing bigger parts. I ended up doing the pass side fender and door and a bunch of smaller stuff with surprisingly good results. 9:1 might be a little too thin, iirc I ran it at around 4-5:1 which was cleaning stuff with pretty heavy surface rust to completely clean in 4-5 days. But that was having to run the shower (read: pump) 24 hours a day. Warm ambient temps help it work faster too, I think it virtually quit working at under 50*F or so. But even at ideal concentration and conditions, it still takes awhile to get the metal 100% derusted. And being an organic material, it does support bacteria growth so it starts to get pretty funky after a couple of weeks.
The black film is iron oxide redepositing on the surface. As others mentioned it's nothing detrimental and does act as a temporary protective coating. It should wipe right off with a good solvent.
Incidentally that Evaporust stuff you can find just about anywhere nowadays does the job faster but it's way pricier and comes already fully diluted. There are concentrates available that make it a bit more cost effective, I got some awhile back that was about $70/gal and I think you can dilute it to make up to 20 gals. I'm just in the process of getting the new shower set up now so hopefully will have some updates in the next week or so