MSK,
Is this only when you are having your periods or all the time? One breast or both?
Ms,
Women often have discharge coming from their breasts throughout life. What makes me a little concerned is that this is the first time for you and the discharge is black.
I believe you need a physical exam by a physician and also a mammogram.
See this link:
http://www.imaginis.com/breasthealth/nipple.asp
Regards....
Doctor (MD)
Hospital Medicine Director