You dip the stainless part in a light acid solution first. Something like concrete cleaner. (It's all in the instructions) Then wash it off and then dip the part in the solution. It basically blues/blackens the stainless. At that stage you can keep it like it is or you can polish off the surface with 3m polishing paper so only the cuts and background are black. You can even use a pink eraser and get a type of French Grey...............It's very effective.
But like I said, you need to play with it first on a piece of engraved scrap because like anything else it has its quirky things that happen.