The friction caused between the skin and the fabric can further lead to darkening of knees and elbows. It may also cause dark underarms. Consuming certain foods as well as the lack of some vitamins may be triggering the condition. Eat food rich in vitamin A, B12, C and E like pomegranates, carrots, beetroot, apples, fish, broccoli, kale and citrus fruits. Olive oil, avocado oil and jojoba oil are good skin moisturizers. They greatly improve the skin's condition.