Any way to get 'er done! Magic Marker works well. Birchwood & Casey brand's product called Brass Black (think gunsmithing stores) will darken brass very nicely. Don't contaminate the bottle with exhausted or partially exhausted solution from a Q-tip by double-dipping--keep the bottle itself un-contaminated by removing a small amount and using that with whatever way you want to use it. Candling works great too. Black paint works well.
Acids and salts and heat call all take away the shine and lustre of polished brass. On nickel plated blades, marker works well and is fast. Blacks, browns, and rootbeer colors all will work. I think the lack of flash, lack of lustre, lack of sheen, is more-critical than the absolute color or shade of color under the lowest and warmest and clearest water conditions. If the water is more cloudy (like cloudy green + full sunshine on the water), the shiny black nickel finish + green prism tape can be excellent.
The idea that absolute flat matte, no-shine black is the only color that will work is an overstatement; black has tremendous silhouette against the surface on blue-sky and white-sky days and gray-sky days. In more-cloudy water with less visibility (like normal year lower Clackamas, or Deschutes) there's nowhere near the need to kill shine as there is in say 500-900 cfs water that has 15 foot visibility; in fact, I thiunk shine and flash of black nickel is at times, MORE-effective than matte black no-shine for summer steelhead and spring chinook.
If you're just darkening a few factory-wrapped spinners, use Magic Marker in black or brown, or use some tape on the back. Paint works too. Nail polish can work too. I usually bake painted parts about 15 mins at 180 degrees.