Hello,
Welcome to ehealthforum and thanks for writing
Black vaginal discharge can be due to:
1)Infection which is fungal or bacterial in origin.
2)Old menstrual blood being shed during ovulation which is normal.
3)Sexually transmitted infection:Chances are present when you have intercourse with an known partner.
4)Any foreign body as tampoon being left inside the vagina which again can lead to infection.
You need to consult a Gynaecologist for evaluation so that the exact cause can be ruled out.Till then follow general hygeinic measures.Avoid vaginal douching,perfume and spray use.Use cotton underwear.
Take care!