I use cold blue until I am through testing & adjusting, then I pull the sights back off. I bead blast, degrease & coat them with one of the matte black thermally cured resins with an airbrush, then bake to cure. This is a pretty quick, easy and (for use on sights) satisfactory finishes.
3 choices available: Hot blue, cold blue and spray and bake.
Cold blue wears off very quickly and depending on the cold blue used, is difficult to get as black on repeat applications.
Spray and bake will chip if dinged, and a cold blue touch-up of the chipped area looks terrible.
Hot blue is fairly wear resistant and can be touched up with cold blue.
Of the three, hot blue is the most difficult for me, as I do not blue in house.
But it is the only finish I use on sights.
The spray & bake that I use comes from John Norrell. I can't find his number right now, but will look this evening. It requires an airbrush, oven and squeaky clean prep work to get the desired results. Chuck is right, it will chip if knocked hard enough. If you don't build up the surface too much with a bunch of coats, you can re-touch with cold blue, or just spray & bake over the first coat. Like Chuck, I don't blue myself. I started using this product so I could offer a quicker turn around than sending out for bluing. I like hot blue better myself, but am almost as happy using Norrell's phenolic resin. I prefer to limit it's use to sights, as opposed to the whole gun.