Ben's Cycle sells a special NJS stem-blackening agent for about $15 but the applicator kit is another $25, so it can get pretty expensive.

If you decide to go the home-made route, be sure to use the K-15 method rather than the K-11 method as there are less postules created this way.

As well, be sure to use all the proper protective gear but I am assuming you know this. There is a pretty extensive thread on the subject in the Mechanics sub-forum, search for that; it used to be a stickied thread here but the debate got rather heated over the merits of the NJS blackeners over the non-NJS stuff and there were quite a few bannings over it, if I recall correctly.

Anywho, good luck and take plenty of pics of the process, it has been ages since we had a good Blackening-Steer-Tube build thread.