The tracer is normally woven as part of the fabric insulation. It looks like hash marks all the way down the loom. In the case of my truck, the insulation is tan with various tracers such as red, black, red and black, blue, etc. The blue is hard to distinquish from black in my case even though the insulation and wiring looks almost new. The blue has darkened.
I will guess that NAT (natural) refers to the yellowish tan color of the cloth loom.
I don't think there would be any reason to label a wire as NAT with BLK tracer. All of the looms are tan (NAT) so the only color that is relevant is the tracer color, e.g RED, BLK, BLU, or RED & BLK
