I'm with MarkG on this one. I touch up score reels all the time, using new black Sharpie with a nice (not mashed down) pointed tip.
When I mess up and accidently get the black ink onto the white part of the reel, I have found that the best way to clean up the blot is by gently scraping it off with an XActo knife blade. It takes a bit of practice, but scraping off the blot allows you to restore a nice clean edge between the black and the white.
- TimMe