Thank you for the information,
The rash and itching in underarm skin, under the breast fold, groin area are most likely due to fungal infection which is known as intertrigo.
It usually occurs due to sweating or moisture.
Sometimes bacterial superinfection may occur.
A review by dermatologist can be taken.
Keep area dry and cool.
Use cotton and loose underwear.
You can need antibacterial and or antifungal cream.
Darkening of the skin can also be seen in adrenal gland insufficiency.
This will need hormonal study & consultation with an endocrinologist.
I hope this information will be helpful.
Please ask if you need any clarification.