I've tried selenium dioxide cold bluing on gun parts, and the results initially looked great. But it rubbed off under normal use so fast as to be worthless. Maybe it was the wrong type of steel and finish (close to polished). I'm also notorious for not being able to follow directions, so there's that. When I'd had enough, I removed the bluing completely just by wiping the parts vigorously with a cloth.
I have parkerized a bunch of stuff, and that's the way I'd go with a carbon steel kukri. Parking works better if you sandblast the surface, but even on a polished knife blade it works pretty good. The phosphoric acid etches the surface. It can be done on a knife by wrapping the blade in a paper towel soaked with solution, then holding it over a heat gun! If it's uneven, use some fine steel wool on the ugly parts, wipe down with acetone or alcohol, and repeat. After done to your liking, rinse it off and soak in a light penetrating oil. Sans sandblasting, most surface imperfections are still going to be visible, though. So you have to finish it well or live with the consequences (i.e., make sure all the sanding patterns go in one direction!). And it won't hold up to abuse as well, but it will also be easy to touch up.
After getting tired of cleaning off surface rust, I did my bandsaw table by covering it with a layer of sawdust soaked in zinc phosphate solution and hitting it with the heat gun. It looks like the deck of a battleship, now. Gun metal blue-grey, and it's near maintenance-free. Lightyears ahead of oil and paste wax. This was by one of the best results I have achieved. Cast iron must take well to parking.
