About 4 years ago, we bought the Birchwood Casey, "low temp" black oxide kit. 190 degrees. Came with 7 buckets, 3 were for rinsing. Two hot plates for the degreaser and the black oxide tank, and a 5 gallons of a rust inhibitor/water displacer, and a test kit to make sure all the chemicals were right. About $600, plus freight.
It was to mil-blah-blah-blah, which is what we needed. Worked pretty well. Got us through the one job we needed it for, saved about $200 plus shipping, plus lead time. Lasted for a few more small jobs after that(probably would have lasted longer if we didn't have an idiot setting it up, and we didn't try to black oxide everything in site). Then we ran out of chemicals, and they are not cheap. The bucket of low viscosity rust inhibitor came in handy many many times though.
Downsides, took forever to heat the 190 degree bucket up on the hot plate, about 3 hours. It was also pretty labor intensive, going from tank to tank to tank to tank, and keeping track of the time in each tank.