Hi,
Welcome to ehealthforum,
The dark discharge that you describe could be due to the use of the hormonal birth control, possibility of infections, old oxidized blood coming out, cervical problems, pregnancy, etc. You should consider getting a thorough evaluation done by your gynecologist to look for the exact cause. You might need to give a break for some time and then get back on the birth control use once your body & hormones get back to normal.
Take care.