Do it yourself "charcoal" blue

[Mark@Sea]

Also known as nitre bluing (you can also get a kit from Brownells) or fire blueing. Gives a beautiful shade of iridescent blue, but is easily scratched or worn off...

Just found this on another forum:

The process for charcoal blue is a fairly simple one : polish, degrease and heat! Heat slowly and evenly, until you get the color. To heat slowly and evenly I use a cast iron jar filled with river sand, I put the prepared metal in and I let it cook in the kitchen for 2 to 3 hours. After two hours, I brush off the sand to get a look at, and when the metal is dark purple, I cut the fire. It'll turn blue as it cools. Oil it and you're done. Don't forget that the obtained shade will darken when oiled! So must give it a few tries before getting it right.

Also found that the temperature you need to reach is 600 degrees (F).

Rust bluing is also something you can do at home, but there is a learning curve to getting it looking right.