Isn't the cassette nickel-plated? You'd have to remove the plating. I suppose you could add a black nickel plating on top but its not very wear-resistant. Or maybe strip the nickel and have the steel parkerized. This would be the most durable solution, I think. Someone might suggest using a Sharpie. Just saying.
I dont know your location, however, every gun shop I can think of in my area offer a blueing service - leave the hardware with them and pick it up 1 week later.
Aren't all the parts either plated steel or anodized aluminum? Hard to blue those. Not trying to be a smart aleck but I would experiment on a part or two with a Sharpie and a light clearcoat.
All the cogs but one are made of steel, however, they don't rust so there must be a kind of platting,,, it's gonna be harder than I though I guess.
Thanks for your help.
Usually, there's nickel plating for corrosion protection. You can easily confirm where the chain has worn the plating off, or if a new cassette, you can scuff an edge. Under the nickel plating is always copper plating. You should be able to see a copper-colored outline along the wear edges.