Friend, when we see black stool it could be a sign of bleeding in the upper gastrointestinal tract. The reason why the blood is black and not red is because the blood is being digested. The fact that this has been going on for so long I would want to rule out parasites. But, it is also important to rule out: Ulcers, infections, a foreign body, tumors, etc. to name a few causes. I don't believe this is related to kidney failure. To determine the exact underlying cause she will need to be seen by a veterinarian for a hands on examination, fecal analysis, blood work and xrays as an initial screening test. If cost is a concern I would look into your local ASPCA or Shelter where they often offer veterinary services at a lower cost. In the meantime, this is what you can do:
1- Do not give things like Pepto Bismol or Imodium. Start her on Pepcid AC (famotidine) 10mg 1/2 tablet every 12 hours. You can buy Pepcid AC over the counter in any pharmacy.
2- Change her diet to rice (50%) and boiled boneless chicken or ground beef (50%).
3- Follow up with your family veterinarian for a hands on examination as described above.
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