Hi Lisa,
Do you mean the area where the mark goes through two differnt areas of colour - ie crosses the boundary of two coloured areas? If so, I used the clone stamp to sample a clean area of the boundary - there is a small set of cross hairs to accurately align the clone stamp tool with the boundary accurately.I then very carefully applied the cloned area so that it was aligned with the boundary in the same manner in which it was sampled. Sometimes you can be out by a few pixels - in which case just delete and repeat.
Not the most eloquent explanation! May be I should have just said "I used the clone stamp"!
Chris