Hello. Thank you for asking your question. My name isXXXXX will try to help.
It usually means there is some digested blood in the stools. The digested blood is what gives the stools the black color. Usually the bleeding is in the stomach or in the small intestines. If the bleeding is in the colon, or large intestines, then the blood appears to be bright red.
The most likely cause would be a bacterial infection or a parasitic infection.
This could be diagnosed with a fecal test at a vet clinic.
If she had a bacterial infection, she could take antibiotics.
If she had a parasitic infection, she could take an oral anti-parasitic medication.
I do not think it is related to her high kidney
test. It is possible, for cats in kidney failure, to have increased acid in the stomach. This could lead to ulcers in severe cases. But she would also have vomiting, lack of appetite, and an increase in water intake and urination.
Until you can get her to a vet, I would give her 1/4 of a 10 mg pepcid ac tablet every 12 hours. The pepcid ac is a human stomach acid reducer. It will help if she has excess acid causing the bleeding.
You can also give her probiotics. Probiotics help put the good bacteria back into the intestines. Sometimes the bad bacteria try to take over and cause an infection and inflammation. So the probiotics help increase the number of good bacteria. A good one to try is "Fortiflora by purina." They sell it online and at Petsmart. You put it on the food
I hope this helps.
If you have additional questions, please reply.