I have that too, my extend to my neck and underarms and things... and there another name for it.. i forgot. alot of people with poc get it but it also part of insulin resistant syndrome., i use that i scrub and the kids dad would say i dont wash... it very imbarrasing.. there was a cream i use that help and taken metformin help, some, it coimming back more now and not sure why..
i hope the best for you..
Washing does not rid of the darken skin because it is not dirt. I have it too, and pretty much I have just learned to live with it. I have noticed since losing 50+lbs, my darker skin has gotten lighter. its still there, but I dont think it will ever go away complety. Being very fair skinned too doesn't help either.