This is probably due to dead skin build up; I had it when I was little, and my first grade teacher took it upon herself to try to scrub it off-- yeowww! Here's what I'd do... take a nice long soak in the tub, making sure that you sit so the water comes up to your chin. Then, after you've soaked awhile, take an exfoliater like Freeman's Apricot Scrub and gently, with circular movements, scrub the area. After you've done this a few times, it will look much better.
How old are you? I noticed that you are from the valley: Sometimes if there is a darkening on the nape of the neck it could signal diabetes. The valley has a high percentage of people with diabetes, especially among the hispanic population. Check it out.
i have the same problem however i've found that using rubbing alcohol on a cotton cosmetic round or cotton ball works great. just saturate the cotton with the alcohol and begin to rub up down side to side. really work that alcohol into the skin. when the cotton is dark with skin dipose and use another. i do this maybe once a week. it doesnt remove every single bit of dark but you will be surprised at the results. let me know how it works