You can mark most black oxide coatings with the power unit set on DC. One problem, though, is the irregularity of the thickness of the coating. Naturally, the thicker the coating, the harder it is to penetrate, and some newer coatings are even harder still.
It is essential that when etching surfaces of black oxide, there is firm contact between the stencil and the piece you are etching. If the pressure is not firm, the part will re-oxidize itself. The result is a deep dark etch instead of a white etch. We have made vast improvements here with electrolytes that tend to stop re-oxidation, resulting in a clear white etch and usually saving some time.
Rolling will not produce good white etches, so use contour blocks for round surfaces. In addition to firm pressure and direct current (DC), keep the voltage low, at five to ten volts. Soak the contour block well with electrolyte and, in most cases, use a deep etch pad. The time dwell needed to obtain proper results varies from one to thirty seconds depending upon the thickness of the oxide coating to be penetrated.