Sure you have long since finished up this saddle but I usually find that really black hard gunk is where some type of hardware rubs on leather. Think rivots or stirrup bolts, etc. I typically use a little plastic scraper to scrap it off back down to leather and then scrub with brush. Often use Murphy’s oil and then saddle soap for the cleaning stages. Might try the ammonia mixture though on the next really greasy dirty one I get!