Hello, I have gone through your query and understood the concern. Please let me inform you that ovulation occurs two weeks prior to the onset of the next menstrual cycle and this is when an unprotected intercourse can cause conception. An intercourse just before the onset of a period is safe. The black discharge you had is most likely to be due to old blood collected in the uterus or cervical canal, perhaps from your last cycle. It is true that pre-ejaculate contains enough sperm, but this is so only at ovulation time, that there is a chance. If you do not find normal menstrual flow, contact your health care provider. Hope this helps.