I use oxpho blue for small touch ups like the ends of pins or bottom of screw slots that may get brightened up during installation. Other than that, I don't depend on a cold blue to do much.
They usually end up expanding a small area you're trying to fix into a large area you now need to fix.
Oxpho blue does have the big plus in that it does not after rust. Many of the other cold blues do including 44-40 that has been around for years.
I think Numrich sells 44-40 now. It does work (blues) pretty well. But will stain some surrounding blue too.
It will rust and cause extensive pitting if not flushed from the surface with warm water after application. A bit of soap helps too.
I've had more than a few guns brought in that were lovingly 'reblued' with the stuff and then packed away in storage.
When next looked at, the gun case was stuck to the gun with the rust in between. I think it's the stannic acid(?) compound in the stuff and others that causes the rust.
Cold blued guns have a distinctive smell to the metal too,,at least the ones with the corrosive agent(s) in them.
Oxpho blue doesn't seem to have that odor to it.
I've never tried the newer "Nu Blue' or any of the other cold blues out there. A couple of gun show dealers I know swear by Nu Blue. That should tell you it might work pretty good.
I rust blue even small parts that need to be blued.
For me,,Cold Blue is for scratches in the bottom of the screw slot after re-assembly.