My underarms were VERY dark due to shaving too much as a teenager ( I had really thick hair). Very distracting and embarrassing. People constantly told me I looked like I had hair under my arms when in fact, there was none. So I took up waxing which lasts way longer. Discoloration didn't go away and actually got worse once when a friend burned my armpits. I had read a lot about baking soda and tried it by itself- not much of a difference. Decided to pair it with lemon juice. Mixed it til it was almost a gel like substance. Worked wonders after only a few days. HIGHLY RECOMMEND. Natural and safe.
Lemon juice rubbed directly onto skin discolorations followed with a very light coating of moisturizing cream. Keep using lemon juice until skin discolorations are lightened to your satisfaction (may be too harsh for sensitive skin). Use SPF sun protection during the day!