I've cold blued a few entire firearms...the key is a thorough degreasing, even application, and a little bit of heat.
Here is a remmy 511 scoremaster (1956yr production) I refinished as a gift. It was cold blued with OxphoBlue (2 applications) after all the metals parts were heated with an electric heat gun.
This was, of course, after ALL the metal parts were stripped of their original bluing (Naval Jelly doest removes bluing!) and all parts were cleaned up with a 600grit wetsand to remove pitting/blemishes.
It's not the best pic, but you can see how dark the bluing is...as well as how even the application is. (This rifle was a $50 rust pit when I purchased it).
Unfortunately, cold bluing isn't usually a resilient to scratches/wear as a hot bluing would be...but it's been just over a year since that .22 was refinished and it still looks like new--but it's only been taken out and shot a handful of times.