Curious if it is known what amount (percentage) of digested blood needed for stool to clearly look black and tarry.
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Hello,
An internal medicine specialist might be able to provide a more specific answer, but, I would presume this is dependent on multiple factors like: age, species, overall health, inciting cause to the problem causing the clinical sign, temperature of the individual, gi transit time, diet, etc. etc. There are too many things that cause and influence this clinical sign. Also, dark tarry stools can occur without blood being present or the cause. I have seen it occur with toxin ingestion, drinking really nasty pond water, etc.
From what I gather, unfortunately it takes quite a lot of blood to make stool black and tarry; way more than for fresh blood to be apparent. It’s not always an emergency but I’d consider it one.