I use speeedball block printing ink, oil base, to darken my engraving. Rub it in to the cuts and then lightly wipe it off the surface with a paper towel until the surface is clean and the ink is in the cuts. let it dry for a day or two and then you should spray something on the surface of the plaque to prevent it from tarnishing like a spray lacquer or even a good pasted wax.
Others I work with also like to rub in rustoleum flat black enamel paint in place of the printing ink