Black marker
If it is black anodized aluminum, you can get a Sharpee permanent marker, fine point, and mark over any visible silver aluminum. Work it into the scratch a little with some pressure on the tip. Then wipe with a clean cloth. It is not super permanent but does a good job and will not wipe out of the scratch easily unless you get some kind of solvent on it. On shiny black frames, it can make a tiny scratch/nick more or less invisible. They make some stuff called aluminum black from birchwood casey, but it leaves a matte finish and I don't think it is very durable for the money, in my experience. Maybe wipe the scratch with alcohol and dry before applying the marker, in case there is any gun or human oily residue there. If it is some kind of shiny black polymer on steel, you would need some kind of paint, I guess. Something real durable and a very fine brush. Testers hobby paint is an example of a paint that is not that durable! They took the good stuff out of it that made it strong, for health reasons.