dark blood is usually nothing to worry about. I have this occasionally and my doc said it was just old blood that got held up and is just now coming out. I was like gushing black and he wasn't too worried about. my uterine lining just took a while to shed. he said he would start worrying if I had fresh pink blood cause that would mean that something was currently bleeding. I also did find out that I had a cyst and endometriosis. with that said it could be something more serious and if your trying to conceive I would consult my doc