Here is what I could find:
Endometriosis is a condition in which tissue similar to the lining of the uterus is located outside the uterus. Usually there are implants of this tissue in the pelvis. When the lining of the uterus bleeds during the menstrual cycle, these implants also bleed. This causes pain and scarring in the pelvis. The other pelvic structures react to this bleeding by becoming adherent to each other so that tubes, ovaries and intestine are stuck together. If the endometrial tissue is within an ovary, that ovary will fill with blood. These are called endometriomas and are cysts in the ovary filled with old blood. This old blood has the appearance of thin chocolate or motor oil. They are also called chocolate cysts of the ovary. Endometriomas are frequently found at surgery for ovarian cysts. http://www.gyncancer.com/ovarian-cysts.html
What are Endometriomas/Chocolate Cysts?
This disease process is also known as Endometriosis of the ovaries. Tiny implants of cells that line the uterine cavity become transplanted and form small cysts on the outside of the ovary. These cysts enlarge and produce Endometriosis of the ovary. They respond to hormone stimulation during the menstrual cycle and produce many small cysts, which may then occupy and even replace the normal ovarian tissue.
These endometriomas are filled with a thick chocolate-type material, which is the reason they are known as "chocolate cysts". When this type of ovarian cyst ruptures, the material spills over into the pelvis and onto the surface of the uterus, bladder and bowel and the corresponding spaces between. Adhesions can develop because of this rupture and may lead to pelvic pain. http://www.alternativesurgery.com/education/ovariancysts.php
) Endometrioma or "chocolate cyst". These are cysts that form when endometrial tissue (the type that lines the inside of the uterus) invades an ovary. It is responsive to monthly hormonal changes, which causes the cyst to fill with blood. It’s called a “chocolate cyst” because the blood is a dark, reddish-brown color. Multiple endometriomas are found in the condition called "endometriosis". Although often asymptomatic, chocolate cysts can be painful, especially during your period or during intercourse. http://www.ovarian-cysts-pcos.com/ovarian-cysts.html