Slow rust bluing is an old process where you let the exposed surface rust, usually by applying an acid to make it rust nice and evenly. After there's a nice even coat of rust, you boil the part, then scrub off the excess with a very fine wire brush or very fine steel wool. Repeat this process 5-7 times and you get a nice, blued finish. Like any bluing, metal prep is key.
Overall, it's not hard, but is pretty time consuming. Midway has a video showing the basic process: http://www.youtube.com/watch?v=VPD3GWqk-yE