How do you get rid of dark marks on bikini line?

That depends: Depending on what skin condition caused the dark marks, there may be some Prescription Bleaching creams that can be of benefit, or some conditions, may be susceptible to laser treatment. Other rashes may require a simple steroid cream to cure the area and eventually cause a return to the normal skin color. See the Dermatologist to get the best answer/cure for you. Best wishes.