Tar stools occur in bleeding in the stomach or upper intestinal tract when there has been heavy bleeding.
If bleeding occurs under 100ml, the haemoglobin is largely broken down by the intestinal bacteria and is therefore no longer visible. In the case of bleeding in the lower intestinal region, the haemoglobin can no longer be broken down, which is why the stool in this case is rather mixed with light to dark red blood (depending on the location of the bleeding).
Tar stools are frequently found in stomach carcinomas or stomach ulcers and should therefore be thoroughly clarified.