What Causes Black Vaginal Discharge?

Hi!doctor i am 20 years old.from last 2 months i was suffering from white discharge from vagiona.i consult with doctor she advised me to take cycloset syrup.after taking this i relielf from it for 7 days.but from last 2 days i am suffering from black discharge instead of white.does it dengreous for me in my future?is it sexual problem?