Abrasion causes this problem (where collar rubs against your neck) and is particularly problematic with dark cashmere. My tailor knew of no cure (including reversing or changing the coat collar), and so I've given the coat away. I had thought that the coat could be somehow pressed differently to hide the discoloration. No dice.
I guess I didn't explain well enough. I meant that the lapels are actually more defined on the edges, because they are lighter in color. No miracle, huh? I really like the jacket, but I guess it has served its time. I'm wearing the suit today, and noticed it when I looked in the mirror.