A small streak of fresh red blood can come from irritation of the large intestine or rectum, while dark or black stool can point to bleeding higher up in the gastrointestinal tract. Diarrhoea, constipation, parasites, inflammation and other diseases can all cause blood in the stool.
Notice whether your cat is also vomiting, losing weight, eating less or changing its behaviour. How long the problem has lasted, and whether the stool has been loose, hard or normal, is also relevant. With recurring problems, the vet may need to examine stool and blood and sometimes go further with imaging or other gastrointestinal investigations.
Book a vet if the blood keeps coming back, the amount is obvious or your cat has other symptoms. Black stool, marked lethargy, repeated vomiting or a cat that stops eating call for a quicker assessment.