I've used that "dye" in my early attempts at sheath making. You get a grey based black (vs blue of commercial dyes) that can be light some times but always a black.

Main problem I had with it was the smell. Once the mix cures it has a nasty smell and it takes a while for it to fade from the leather. BTW you can also just soak steel wool in vinegar for the same results.

Another good old fashion dye is walnut husks. If you google it you will get a couple good recipes for a very nice brown dye.

Jim