Pretty much anything that has an oil base, (Mink Oil, Neatsfoot oil) is going to darken the leather. They will however soften and waterproof the bag. Use of the bag will again lighten the bag, but what is happening is the oil is slowly being worn off.
Used to use neatsfoot oil like religion in the winter in Nebr. would come in after hunting, running my trapline and pull off wet boots. I 'd dry by packing with newpaper, and setting by heater. Then pull out wet newspaper, paint on a coat of neatsfoot oil with paint brush and set back by heater. This went on all winter after a few treatments, boots would begin to get more water proof and require not nearly as frequent coatings. Between treatments leather begins to lighten each time.