Have I over- or under-cleaned my cast iron dutch oven?
The black areas look like seasoning, which is essentially burnt-on oil/fat. You want that (though probably you don't want proteins or sugars so much, but I'd guess you've removed them now). Seasoning is somewhere between a dull matte black and semi-gloss black, depending. Wet with the tiniest bit of oil (e.g., wiped on with a paper towel), it should turn semi-gloss to fully shiny.

The metallic gray is bare iron; you need to season that promptly, before it rusts (apply oil, heat, repeat a few times); we have questions on how to season a pan already.

Personally... I'd just attempt to season the whole thing at this point. If the seasoning fails to adhere to some of those black areas, then I guess they need more sanding. But I doubt it.

PS: A good pair of gloves should protect your hands from steel wool.