She had been on long-term suppressive antibiotic therapy with Minocycline 100 mg twice a day.
Despite the dyschromia, It’s recommended she continue taking minocycline for life because of her previous endocarditis.
Although the mechanism is unknown, minocycline when oxidised turns black. It can discolour skin, bulbar conjunctiva, nails, and teeth.
Sunlight and concomitant use of oestrogen, amitriptyline, and phenothiazines can exacerbate hyperpigmentation. Resolution of the hyperpigmentation takes months to years after discontinuation of therapy and can persist indefinitely.