If you are just looking to darken it, and don't mind a black/gray color, try Radio Shack's PCB etching solution full strength; I used it diluted to do a forced patina on the D-2 part of my Rake blade (per suggestion here on Bladeforum, and it worked great!); when I cut the foil seal out of the bottle with my Rake, it stained the tip a bit, so it should work well for your pocket clip.
Sorry for taking so long to comment, I am fairly new to the forums, and am still working on getting through all of the old Kershaw/ZT threads!