Idk about the black stuff, but that looks like you used a rust converter on the rear axle. Certainly doesn't look anything close to the amber cosmoline I did my trucks chassis with.
Also never apply another rust inhibitors that are lanolin based over petrol based ones. you can get a reaction like a buddy of mine did when he sprayed cosmoline rp-342 over fluid flim he'd done a week before. His taco had the drips, and had to be done completely over.