I've been considering using permanent black marker.
I use the permanent markers (Sharpies) a lot just for marking/layout,
etc, while making jewelry, rather than as a permanent color on the
work. Frankly, I don’t find it all that durable. While it’s water
proof, handling the metal rubs it off quickly enough. It’s a lot less
durable than, say, the oxidiation on silver from sulphur compounds.
I’d suggest paying some actual money on a product, and getting the
commercially made antiquing paints sold for darkening gold. Commonly
used on things like class rings, etc. Several dark shades of color
are available (black, dark brown, dark green, etc.) These paints put
down a thicker, much more durable deposit. I think it’s an epoxy
based paint (solvent based, though, not a two part epoxy or anything
like that). Put it on, let it dry somewhat, or all the way, then
highlight with the solvent made for it, or with plain zylene. You’ll
find this stuff performs a lot better than magic marker in this use.
And testing such a product by just sitting it on the bench tells you
nothing other than that it doesn’t fade in the light. Instead, put
the item in use, wear it, for example. If the surface is porous, or
deeply textured (sandblast), then perhaps marker might be able to
hold on fairly well. But on normally finished metal? Not as well as
I’d like, at least. Your needs may vary, of course, but that’s my
take on it…