I have some information on darkening metal stampings that may be useful to you. Many have complained that the black added to the stamping does not last, regardless of whether they use paint pens, enamel paint, or Liver of Sulfur. This is especially true for stainless steel, which doesn't react to the Liver of Sulfur and does not hold black ink/paint. Customers also want a safe product that won't pose a risk if it comes into contact with food. I researched and contacted some companies regarding their products but found nothing promising.
Then I remembered watching videos of clock makers darkening the letters on the chapter dial with a product called "engravers wax." This got me thinking that black Crayola crayons could be an inexpensive alternative to this product. To test the technique, I used a stainless steel fork. The upper stamping was filled with a black paint pen, and the bottom stamping was filled with black crayon. I submerged the fork in alcohol (a solvent) for 2 hours. The result? The top stamping lost some black, while the crayon-filled stamping still looked new. If you choose this technique, use Crayola crayons because they are non-toxic. Cheap Chinese crayons cannot be vouched for since we cannot know their ingredients.
Here's how to do it: Heat the metal piece from the underside. You don't need much heat. Touch the engraving with the black crayon and let it melt into the grooves. Let it cool. You can scrape off the excess with your fingernail and remove the remaining surface residue with a paper towel and some alcohol. Remember that since it's a wax, it is susceptible to heat. If your customer plans to use a stamped spoon for their morning coffee, tell them to stir quickly and then wipe the spoon off. Don't let the spoon heat up; do not use the dishwasher. I scrubbed the crayon engraving with Dawn dish soap and a toothbrush, which didn't affect the crayon. They are fine to wash by hand but do not submerge them in hot water for any time. I suggest making up "care instructions" to go with your stamped items on how to care for them if they need washing at some point.
Crayons come in a rainbow of colors and are also a great way to fill your stampings with COLOR!