Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek emergency care now if black stool occurs with:
Vomiting blood or coffee-ground-like material
Fainting, confusion, severe weakness, or difficulty staying awake
Marked dizziness, especially when standing
Trouble breathing, chest pain, or a racing heartbeat
Severe or worsening abdominal pain
Pale, cold, or clammy skin
These symptoms can occur with significant blood loss.
Do not drive yourself if you feel faint or very unwell.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Arrange same-day medical assessment for an unexplained black, sticky, tar-like bowel movement.
This is especially important if you take a blood thinner like warfarin or apixaban , have a bleeding disorder, or have a history of ulcers, digestive bleeding, or liver disease.
3
Does any of this apply?
Book an appointment
Book a routine appointment if the dark colour continues after the likely cause has stopped, keeps returning, or comes with ongoing tiredness, abdominal symptoms, or unexplained weight loss.
The article also says
If a known food, iron supplement, or bismuth medicine seems to explain the colour and you otherwise feel well, monitor the next bowel movements.
Every sign above is quoted from the sofidex article on Black or tarry stools. If you are not sure, err upward — it is always reasonable to be checked.