Hello and welcome I share your concern. Blackish color stool is a matter of concern so I would advise you to go for stool test for ova, cyst or protozoa and occult blood to rule out blood in stool. Looking at your description it seems to be infective diarrhea so I would also advise you for course of antibiotics like ofloxacin or ornidazole or its combination under medical supervision. Yes, you can take probiotics which helps by strengthening useful bacteria of the colon, you can also take curd and yoghurt. I would advise you to take plenty of fluids like ORS, fruit juice, water mixed with salt and sugar to counter dehydration and electrolyte imbalances caused by diarrhea For cough you can take honey mixed with ginger juice or cough syrup for symptomatic relief. warm saline gargling and steam inhalation will definitely help in relieving cough. Hope I could answer your questions, please feel free to ask for follow up question, if you have any more queries, I will be happy to help you. Best wishes