My period blood is normally bright red but has been a black red the whole time. It has lasted for three weeks and counting. What should I do?

Old blood. the darker the blood is basically the older it is. Most likely you aren't ovulating regularly and as a result have prolonged breakthrough bleeding. Its time to see a gynecologist. If you don't have one at least call your family physician tomorrow and explain the number of days you have been bleeding so either something can be done or they get you in with GYN.