Linseed oil alone, if boiled with lead, will go black over many years. Usually it's a somewhat translucent black, but very black nevertheless. If it accumulates dirt, dust and who knows what else in multiple layers it will be relatively opaque - 'paint.' I've seen enough old Winchesters covered in what looks like dried black coffee to know that frequently if a gun was rubbed down with oil, it ALL was oiled - barrel, buttplate, all of it. If someone wiped a gun down with leaded linseed oil and stuck it up in the rafters for a hundred years, between the lead and the oil (which in and of itself will darken quite a lot) and the heat and the dust, you'd end up with black paint.