Opinions of which cold blue is the best, not so much in ease of application, but I am more interested in final appearance and durability.

I have had good luck with Birchwood Casey Perma Blue. Just make sure you folllow the instructions or it won't come out right. I've had the best luck with putting the handgun(stripped and unloaded) into the oven at warm temp. just to get the gun warm. Don't try to cook it, just get it warm.

SAV308

I second birchwood/carsey perma blue, so easy to use even I figured it out.

Oxpho-Blue from brownells.

Here's what I do and get a great almost black finish. Disassemble clean parts of heavy krud and grease. Then into the dishwasher with some cascade, soon as it done, wear latex gloves to keep finger prints off, get the parts hot with a hair dryer then use cotton ball and q-tips to cover everything with the bluing. Then I dry again with hair dryer, then back into the dishwasher again, and repeat the process. Finally rinse the parts in plain water and dry. The metal will be super clean and will soak up your favorite lube like tri-flow. It will polish up to great finish. Last for years on my regular match guns that get a lot of use.