If you have a piece of steel or iron you would like to turn black, and you do not want to paint or powder coat it, using a gun bluing solution will leave your steel with a dark black color. Cold gun bluing involves the application of a selenium dioxide based compound, which will permanently color the surface of ferrous metals while also protecting against rust. This can be applied safely in the comfort of your own home, and does not require any heating or specialized machinery.
Clean the piece of steel you wish to blacken using some solvent-based degreasing solution. The steel must be free of any dirt, oil or other contamination before proceeding. Check for cleanliness by wetting the cleaned steel with water. If it beads off the steel, it requires more cleaning. Otherwise, proceed to Step 2.
Apply some gun bluing solution to a clean piece of cotton cloth or a toothbrush, and rub it onto the surface of the steel. The longer you apply the solution, the blacker it will become. If you have enough solution, and your piece of steel is small enough, you can submerse the steel into the solution.
Wet another piece of cloth with some new or used motor oil. When the desired blackness has been attained on the surface of the steel, rub in the oil-soaked cloth to stop the reaction.
Buff the blackened surface of your piece of steel by polishing it with a piece of 0000 steel wool. Reapply the gun bluing solution to further blacken the steel, as you wish.
Repeat the process after buffing, to further darken the steel to your liking.
This solution will not work on aluminum, stainless or other steel alloys.