What could cause Black Stool?
Blood present in the stool is the main cause of black stool, usually from a polyp or a bleeding ulcer. But in your case no evidence of blood was present, so it is probably just due to the diarrhoea from a possible infection. Licorice can also cause the stool to be black, have you eaten this recently? Just let everything settle down and stop worrying too much. I am sure the Dr is doing all the correct investigations.