I knew a very talented engraver, who once worked with Colt, and had him do some high end engraving on a British double rifle for me. He too darkened the engraving background, using what he called ink. I took it to mean India ink, or some sort of permanent ink. It worked well, and is still on that rifle. Engravers would know for sure. Visit some of the engravers forums, and pose this question there; I am sure you will get appropriate answer.