Dark skin is either genetic or caused by pigmentation due to over-exposure to the sun. The pigment producing cells, melanocytes, sometimes go into excessive production of pigmentation. This leads to uneven patches when certain areas start producing excessive melanin permanently leading to permanent tanning, while other shut down production of pigmentation when they can not cope up due to the effects of over-exposure. The best way to combat this kind of patchy discoloration is to avoid exposure to the sun. If you have to step out, make sure you have applied enough sunscreen lotion of a reasonably high SPF factor and if possible carry a parasol with you to shield your face from the direct sunrays.
If you already have discolored patches on your face or else where on the body (neck, back, arms or legs), you can try applying lemon on the affected area and washing it off after 15 minutes. You can do this daily, since lemon acts as a natural bleaching agent. Alternately you could do this and leave the lemon juice overnight, washing it off the next morning when you bathe, or even bleach the affected area once a month. However do not bleach when you are pregnant since the chemicals could be harmful to the baby. Another way to lighten the skin is to prepare a tonic with 3 tsp borax powder, 2 tbsps glycerin and 2 cups rose water that you can apply to the affected areas. An overnight cream can be prepared at home by beating lanolin and butter in a double boiler over boiling water, and applied to the affected areas in a circular motion. If the discoloration is excessive you could consult a dermatologist who could prescribe appropriate treatment, include mild chemical peels.
answered by G M