Can medical coal cause black stool?

Medical coal - what do you mean? Most coal products are like Coal Tar and are topical cream and the like. Or have coal elements inside other drugs. If you have black stool this normally means your are taking iron supplements or you have blood on your stool. If you are not taking iron I would see a doctor as blood in your stool needs to be dealt with.

UPDATE:
Right Activated charcoal is not coal so the answer is yes. It is given for a number of reason of acute poinsoning (but NOT including Cyanide, Lithium, Alcohol or Iron tablets). Taken by mouth the side effects can be Black stools, Black tongue Vomiting and/or diarrhoea or Constipation).

Interactions. Activated charcoal may reduce or prevent the absorption of certain drugs. This may include drugs such as: Acetaminophen(aka Paracetamol or Tylenol) , Digoxin, Theophylline and Tricyclic antidepressants. So do not use activated charcoal as a supplement if you take these medications. Activated charcoal may also reduce absorption of certain nutrients.

Absolutely.

Yes