Was intrigued by this simple and harmless (no noxious gases or chemicals) means to blacken a knife blade, so I tried it.
As said, you are reversing the rust removal method and actually placing black oxide onto the knife blade. My initial 13VDC 0.9A wall wart power supply gave up the ghost halfway through, so had to get creative in finding a suitable DC source, made a cable to run off the aux power outlet in the back of my 4Runner.
Every 5 or so minutes I pulled the blade out (yes, I could feel the electricity running through the salt water solution!) and wiped off the thick black slim that formed on the steel, leaving me with this after about 20mins.
I then put a couple coats of cold gun bluing on the blade, and it's really black now! And tough too, 2000x sandpaper doesn't scratch it, coarse ScotchBrite pad doesn't either.
I'm quite impressed overall, both in that it's a tough finish, can acheive either a grey or black finish, and is stupid simple (I like that).