While discoloration could be due to irritation, it can also be from acanthosis nigricans, which is related to blood sugar. I’d suggest mentioning it to your doctor and asking about getting your blood sugar levels tested. It’s better to know if that’s an issue sooner rather than later!
CosRX BHA can be good for ingrone hairs. It won't fade dark spots. It's not a bad choice in that it's pretty mild. Don't over use.You could try a mild AHA toner like Pixi Glow.
Check out this video by Dr. Dray and gog about 18 minutes in. She discuss Palmers Fade Milk and serum for those kinds of body discoloration. It has some hydraquinone.
Besides actives, serums with niacinamide, licorice, c, arbutin, tranexamic acid might help. Hada Labo's new Shirojyun line expecially the premium in the dark blue bottles. .
Yeah, like the other reply said, the discolouration in those areas is typically due to hormones.
Though I did see slight brightening, probably because I took off a layer of dead skin or something.