11 years' postmenopause, I've a thick, dark discharge, with a watery substance, at least 2 times/monthly. Ob/gyn tested last fall; showed nothing. Frequency's increased over a yr; mom had ovar cancer?

Not normal. Post menopausal spotting is related to a 20% risk of endometrial precancerous or hyperplasia at least. You need a pelvic ultrasound., a good gyn exam with pap, and an endometrial biopsy. If this fails to show anything then a d;c is needed to remove any built up tissue from your uterus.
Vaginal discharge. Your gynecologist is the best person to advise you here, since have no means of examining you or the discharge. If you are not fully satisfied with their opinion or simply want a second viewpoint, the ohio state university medical center has an outstanding ob/gyn department and would check with them. Best of luck and hope all stays well.