My vagina blood is black what could b wrong?

Not sure: Old blood can appear dark as you describe, but to describe this as a "black" discharge is a little concerning and deserves evaluation with an ob/gyn. Also, your history is a little odd in terms of your prior surgery and irregular periods. There are obviously more details that should be sorted out in person with a doctor.

Nothing: Black blood is just old blood that's lost it's color. Difficult to tell how old, could be from last cycle or from breakthrough bleeding. It can be scary (i remember when it first happened to me!), but nothing is wrong with your body if you're having regular monthly cycles. Best of luck!