I agree with Ticked...
Many dr's claim to be specialists but few truly are...so make sure your dr. is truly a specialist...if the disease is not excised leaving "clean perimeter of tissue" around the endo lesion and cutting it deep to the root it can indeed still cause problems...there are many variations, colors, appearances to this disease and only the true specialists recognize all stages because of their extensive training... if one little bit of endo is left behind it can continue to cause pain...think of it like a little grain of sand in your eye...tiny...but causes excruciating pain...so if just a little bit was left behind it can cause pain...good luck!
The black blood you are seeing is old blood. I would suggest possibly trying a nother BCP to see if that helps with the irregularity. Are you taking the pill everyday continuously or like someone normally would with the sugar pills?
Endometriosis can grow back if it has not been removed properly.
What is the name of your surgeon? I can look him up and see if he is one of the many specialist that are out there that specialize in Endo. Some doctors say they got rid of it all but even if a tiny microscopic cell is left behind it will continue to grow.
Besides the painful sex do you have any bladder/rectal or bowel symptoms as well?