The "common" black dye of the day was iron dissolved in vinegar, dependent on the tannage it goes from a very dark brown to a true black and blacking was a common finish especially on harness, it was not just brown that got dirty/oily - that statement is based on researching the actual methods used.
This recipe is from a 19th Century harness maker's manual but is the same method that was used much earlier:
VINEGAR BLACK
For giving color to the grain of leather there is no blacking that will at all compare with the well known vinegar black. This may be made in various ways. The simplest, and, without doubt, the best, is to procure shavings from an iron turner (note: some folks today get the turnings from brake drums, but any chunks of iron will work - the smaller the better as they will dissolve faster) and cover them with pure cider vinegar; heat up and set aside for a week or two, then heat again and set in a cool place for two weeks; pour off the vinegar, allow it to stand for a few days, and draw off and cork up in bottles. This will keep for a long time, and, while producing a deep black on leather, will not stain the hands.
For the deepest black, apply a bath of strong black tea first (this increases the tannins) and let it soak in good, then apply a generous amount of the vinegar black. Let set for about a half hour and then rinse with a mix of baking soda and warm water, about a 1/8 cup soda to a half gallon of water, apply let set for a few minutes and then rinse off. While still damp apply a light coat or two of your favorite oil/conditioner front and back. Once dry top coat as normal
Experiment - I test a piece of each new side of leather without oiling to see how well it takes the blacking, if need be I'll do a second black tea mix to darken, then apply the oil which also helps darken. I've been using the above method to blacken veg/bark tan leather for over 40 years.
This one of the easiest blacks to use and does a MUCH better job than any of the commonly available black dyes plus unlike black dyes it won't rub off since the black is due to a chemical change. Unless leather is vat dyed for a great length of time the color can and will scuff and show the underneath but that's the nature of the "beast" even in the past.
I don't have any examples of bags but here is one of a gun rig that I made and dyed with vinegar blacking:
it also "ages" well - giving that "rusty" black look -
Another period "blacking" was/is logwood dye...