Part of the problem is that the dark color you see is probably rust and not just dirt. Unfortunatly no amount of cleaning will remove the rust completely. Musician's swords are not particularly valuable so I would not hesitate to polish the blades. I did it to a coulpe of mine and they cleaned up real nice. The pitting was not very deep in my case. It does take a bit of elbow grease but it is easy enough to do.

Bill G.