[Zeke]

If the part is plated the ER will not remove the plating. So the outcome is not always a ugly dull gray. It depends on the alloy. It it rusts, then ER will remove the rust. Some SS rusts but it comes back looking like SS. For sure it has to be ferrous. Ball bearings have a lot of chromium in their alloy. If they happen to rust they are useless, but the point is that chromium doesn't blacken much at all. Not talking about chrome plating.

On Practical Machinist someone put a piece of steel that had a fresh cut at the end in ER and while the rusted surface was blackened with carbon, the cut was somewhat bright. It was also pointed out that once the rust is gone what you are seeing is result of what rust does, i.e., pitting even if micro pitting. So the original finish is lost.

I did a lot of reading about ER months ago. Many have come close to reproducing the formula using anything from beet juice to molasses (a known rust remover). There is a lot more to it than combining a ratio of beets and molasses. But it's pretty well known that the compound is organic.

You can get decent results with a home brew, but so far not quite the same.