Have you recently imported top soil? Could there be bits of broken glass in it. Black poop plus lethargic behavior may indicate internal bleeding. The bleeding coagulates and is pooped out black.
If you suspect this could be the issue, give your hen uncooked egg whites to eat. Syringe in into her if she won't eat it. This will create a super slimy "wash" that will float sharp bits of glass out of the intestines without doing any further damage.
If this has been going on for very long, your hen could be weak from loss of blood and suffering from bacterial infection. Treat for blood loss with warm sugar water and electrlytes plus Poultry Nutri-drench, followed by plenty of fluids laced with sugar.
Another cause of black poop is decaying vegetation. If your hen has been scavenging in a compost pile and eating things found in it, she may have a bacterial infection. It must be treated with an antibiotic.