The cold bluing and the way it looks depends on the alloy used and it won't give you rust protection. It may or may not blend with the rest of the rifle. Oxpho Blue does a good job on heated carbon steel but not as good on nickel or other alloys when blending to some existing bluing for example.
On a muzzle, depending on which one you use, it may wear pretty quick. Do you need it to reduce a chance of the crown flashing on a hunting rifle or just hate the look of bare steel? Then I would cold blue it. It won't wear very well on the threading if you take muzzle devices on and off repeatedly.
I would also go to Brownells and read the reviews of each of the cold blues as some brands may blend better colorwise than others.