Blood in the stool is a sign of gastrointestinal tract bleeding. Dark stools means the bleeding is higher up and the blood is partly digested.
Red blood means bleeding from lower in the bowel.
I would not delay I'd get her in to see her vet.
It may be she has a tumor or ulcer that is bleeding.
This isn't something home treatment would be a good choice for. With the history of other issues it may be time for you to evaluate her life quality but there is no way to know that without an in person vet exam.
Hope this helps you!
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