That material is probably a two part baked on stuff, for AR's and such. It has no place on a ML. I would not take any more gun projects to him.
Neither does a high gloss blue, ....IMHO. The half used all day sucker look you will get from a modern gunsmith is completely out of place. The polishing they do can do permanent damage. And, that is what you will usually get. The flats will be all messed up and rounded.
The only thing these guys can do, for a decent price is to bead blast it and then hot caustic blue it. Bead blasting will take your paint off. That process actually looks reasonable. Like an old time rust blue. If you are very specific and the guy is not a hack, you may come out OK.
These days many people who call themselves gunsmith have not the first clue, beware.
For this situation, the best course of action is to re-polish to #320. No finer is needed or productive. Back the wet dry sandpaper with an old file. Start at 120 and progress to 320. Birchwood Casey plumb brown gives instant results. A decent cold blue like Brownells Dicopan will well if you want black. Apply it with de-greased steel wool.
I used to do my prep work and then have the local , non hack, gunsmith do a hot blue.
Truth be told, muzzle loaders are supposed to be browned, it is some kind of law.............I think? :grin: