The problem with using metal as the block is that it cross plates when in the AC mark mode.
In the DC etch mode, the steel is stripped from the blade and deposited in the felt and on the cathode ( negative electrode). If that is a metal, the steel plates on it. If it is graphite, it just sits on the surface.
In the AC mark mode the process reverses sixty times a second, moving the steel from the blade and then plating the steel ( now oxidized by the electrolyte) back into the same place over and over again. If the steel is just in the felt and siting on the surface of the graphite, it deposits as a dark iron oxide.
If metal is used as the cathode, then that metal is stripped sixty times a second and deposited in the mark, too.This could also cause a funny "blush" or "halo" around the mark. Additionally, a steel or brass cathode would slowly become contaminated with a coating of oxide and need to be sanded smooth to avoid creating a resistive barrier in the process. Graphite just needs to be wiped off every now and then.
How to get a deep and crisp mark:
Use an etcher with enough amperage and voltage to make a deep etch. Variable voltage is a very significant advantage.
Use a commercial ( or at least proper) electrolyte. Salt water and coke may work, but a $7 bottle of the right mix of chemicals will almost always surpass that in quality of the etch.
Use a larger than the mark graphite ( carbon) block and a dense felt pad. Sure Q-tips and cotton balls will "work", but the results won't be the same.
Time each etch. Keep a log or remember. Don't keep lifting the stencil and peeking to see if it is working ( unless you really want a bad looking mark).
Best tip I can offer - Do a test etch before you ever etch one single blade. Do one on stainless and one on carbon steel. A good choice would be 1084 and CPM-154 unless you almost always use other steels. If you have a preferred steel, use that steel. HT the steel and sand the surface the same as you do your blades. Set the voltage to 24V ( if you have one voltage, just use that) and do a series of etches at 30, 45, 60, 75, 90, and 120 seconds. Lift the pad every ten seconds and put it back down again ( this prevents gassing, which can make a irregular mark). Switch to AC and mark for fifteen seconds, lifting and replacing the pad every five seconds. Lightly wet sand the marks with 600 grit paper on a hard wood block. Rinse well and pick the one that looks the best. Now redo the series ( if possible, just below the first marks) at 12V, sand and rinse. This should tell you the differences between voltage and time to decide the best combination for a deep and dark etch with the minimum of "blush/halo" or fuzziness. There can be a lot of variation in etching some steels, but this test will give you a starting point and a starting length of time for most basic marking.
I etch carbon at 12V and stainless at 24V. I etch carbon for at least 45 seconds, stainless for 60-90 seconds, and mark all for 15 seconds.
