I tried doing the same, but I think I used to coarse of a grit (600) sandpaper. Then I used some Smith’s leather balm to help darken it a bit. Now my saddle brown case looks rough and not in a good way. I’ll end up buying another one from Target.I took the plunge and bought a saddle brown case for my iPhone X in September 2018... almost exactly a year later, I upgrade to the 11 Pro Max, and got the same saddle brown leather case.
Comparison photo is attached.
I expedited the X case a slight bit, by taking some 800 grit sandpaper and lightly removing the coating from the factory, and then applying leather balm over the case for two days in a row.
I’m going to let the 11’s case naturally do it’s own thing.