You can use cold bluing, and yes, it will turn the brass dark, but you can polish it back to the tone you want. If you don't like it, you can polish back to shiny. Cold bluing, and 0000 steel wool will give an aged patina, especially if the rifle is dark stained. If the color is more on the red side, then a little brighter polish looks better, I think. Cold bluing will wear off, but can be reapplied as needed. Some bluing works, better than others.