I was recently diagnosed with gastritis and two internal haemorrhoids through a colonoscopy and a camera down my throat. could either of these cause dark stools from time to time or could it be due to eating black beans or being dehydrated?

Gastritis - if it causes bleeding can cause black stools, Internal hem. cause red colored blood. Dehydration and blackbeans will not cause black stools. Dark and black stools are different. Darkness can de affected by meds eg Iron and foods. Get stool tested for blood and ask your GI doc to explain any questions

The gastritis would only cause black stools if it caused internal bleeding, which is unlikely. Black tarry, foul-smelling stools are a medical emergency. Dark stools, on the other hand, are frequent and normal, often caused by diet and by certain medication‘s, such as Pepto-Bismol or iron. Hemorrhoids do not cause dark stools. See your G.I. doctor if you have black tarry stools.