I don't know if its the same but I had a problem with dark knees & elbows, my mom suggested scrubbing, although skeptical I found out that using a plastic dish scrubbier, you know the ones that come in a pack with different colors, well using one of those to exfoliate the dead skin (which happened to be the dark skin in my case) helped lighten & smooth the darkness from my knees & elbows. And as embarrassing as this is to mention it helps lighten dark armpits too:)
You can go to an esthetician. They do chemical bleaches to all areas on women's bodies. Many porn stars go to get their anal and inner thigh areas bleached. Its an in office procedure that is almost instant though in some cases you need follow-up treatment to get it completely gone. Call a dermatologist - many of them have an in-house esthetician who works with them. I hope this was helpful.
For dark skin you can soak a regular washcloth in lemon juice wring it out not to much and place it in the darkened area reguraly until the skin fades to desired color. This works because lemons are a natural bleaching agent and should help reduce the darkness of the skin. Also if you are allergic to lemons you can use potatoes [russet works best] and slice them in thinish slices and place them on the darkened area reguraly.