IMO, you can apply it using a Q tip or a cotton swab. Wash it off between coats and apply several of them to help make it look uniform.
I think you will have the best luck with the Perma Blue if your barrel and hardware is made out of low carbon steel. The Super Blue is actually made for use on modern guns who are made out of high strength alloy steel. I've not had good results with Super Blue on the low carbon steels.
Both of these blueing solutions create the blueing by laying down Selenium compounds. The deposited material is not very hard so it is not resistant to abrasion. If you rub it out with steel wool or other harsh pads, it will frinkle up the surface more than help it. IMO, your best bet is to use a wet paper towel to "rub it out".
These solutions work best if you degrease the surface either by washing with strong soap and water, or by using disk brake cleaner, then washing the surface.
Do not oil the surface until you are sure it is as good as it gets. Then use a good gun oil like Sheath or Barrier to protect it. The oil will also darken it some and make it look better.
As the others have mentioned, there are more historical ways of changing steels color but if you are wanting to use a quick method, this is the lowest cost option.
As with most quick, low cost methods, it probably won't look as good as the finish modern factory reproductions have so don't expect too much.
I can hear the others...WHAT?...Zonie knows something about that unHistoric stuff?
I wouldn't be without it.
For touching up tiny nicks and darkening small areas like the V notch in a sight after sighting in a new gun, it can't be beat! :grin: