Dark colored liquid definitely doesn´t sound good and you need to see doctor. Monday is good, as it´s very soon.
Generally speaking, dark colored liquid from a nipple can mean breast cancer, but, there can also be other explanations, which the doc I´m sure will start to examine tomorrow.
Cancer I think is a very rare possibility, because you haven´t been on HRT that long.
Good luck and let us know how it goes!
But definitely seek medical advice tomorrow. If your endo is busy, then other doctor.