We use the Black Oxide finish and are very pleased with the results.
There is no smut problem like the cold blue products made by Birchwood, and the color is a beautiful deep black.
We take great care in not getting any contamination of the surfaces of the parts then heat them to about 125 - 150 degrees prior to blackening.
http://www.shopdawg.com/products/SD1009.html
KL Bohn
Shopdawg.com