I am also a sufferer of this condition, and it sucks, I've had it since childhood. It is a very noticeable color difference between my predominantly brown skin and my black neck. That said, these are the things that are said to help -
I have had this condition since childhood and I'm now in my early 30's, there is a girl on Youtube named Divaslimsdown who also had this condition and it faded to normal w/her 100+ lb. weight loss. I am hoping for similar results, I've used glycolic and lactic acid over the years but not consistently, and haven't really tried the other methods. I will definitely post if weight loss appears to rid me of this condition, it usually makes me feel worse than being large.
Also Omega 3 fatty acids (found in cold water fish, walnuts, flaxseed, grass fed beef, etc.) may help as well. Right now for supplements I am taking Natrol Cinnamon Chromium Biotin tablets, Deva Vegan Hair Nails and Skin vitamins w/Biotin, Deva Vegan Flaxseed Oil Omega 3 supplements and I drink a little apple cider vinegar everyday mixed with water to lower my blood sugar. I have never been diagnosed with diabetes but I am insulin resistant and have PCOS so that with calorie counting will hopefully alleviate the problem in time.
Also please check out Using trichloroacetic acid in the treatment of acanthosis nigricans: a pilot study here is the link (if it is allowed it appears to be a government website) -