What should you do when bleeding under your toenail causes a black line to appear?

That depends on how confident you are in yourself. If the bleeding has progressed to the point that it is causing severe pain, you can clean the area as best as possible, heat a pin until it is red hot, and touch to the nail over the blood. It will burn through rather quickly, creating a path for the blood to exit from under the nail. I would caution you however, to do this only if you have no other choice. The best bet is to get to the Dr.'s office and let THEM do it!