I am just a hobby builder, and only build for myself and a few buddies, so my way isn't for a production run, by any means.
I paint them with a black sharpie, heat set it with a hair dryer, shoot on two coats of Rustoleum 2X Gloss clear, and then a final coat of Rustoleum 2X Matte clear. I wipe the blades with clean acetone first, to get rid of any oils. The sharpie is solvent-based, and sticks to the clean metal well. Just be sure it's heat set, so the Rustoleum doesn't make it run too much.