Thanks for the help. I tried denatured alcohol and it did nothing. Then I tried Acetone. That removed stuff. I cleaned as much of the outside as I could, then soaked the holster in vinegaroon. There are some spots on the back that aren't black, so I will try to darken those. The holster is still quite stiff, but the inside is a nice black color.
I'm not sure what will happen when I try to apply the final finish of Resolene.