Your question was , I believe, should you see the doctor right now, or wait.You should not wait.
The degree of bleeding at age 75 can not be predicted from one black stool.
Be safe and go to urgent care or the ER now, get observed and checked out.
I am a US, board ceritified doctor of internal medicine and have worked with patients such as yourself for over 20 years. Board certification means I am current in my practice and knowledge with the American Board of Internal Medicine.
This could be a stomach ulcer that has bleed, and it might progress, just as one example. This could then become a critical emergency.
The stool needs to be tested, your vitals need to be taken, and then decisions need to be based on exam now, not arranging for an exam later.
Let me know if you have more questions.
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