Dark Blotch On Undied Leather

[zuludog]

I made a sheath from 3mm undyed veg tanned leather, which had a similar colouring to yours, and treated it generously with Tandy's Prime Neatsfoot Oil. I was quite disappointed when it formed a few random bark blotches similar to the one you have illustrated. After a day or two it evened out to become much more muted - the pale areas with little oil became darker, and the dark oily patch became lighter and they sort of merged into each other. A couple of weeks later it had become an almost all - over dull yellow or mustard colour with just a bit of mellow shading

In fact people have admired and requested that colouring as they say it is a nice change from the usual dark brown or black. It goes well with a pale wood handle, like hawthorn, beech or yew

You could add even more oil so it's fairly well soaked and even, and see what happens. Or if you're a bit nervous, try on some scrap first. Soaking the sheath in oil and kneading it with the knife in place helps to form & fit them together.