I have had diarrhea for the last few days and its dark black with brown foam that floats at the top of the water in the toilet. Should I be concerned?

Dark black? Black (not bright red) stools can indicate bleeding in the upper GI tract. This needs to be checked out by your doctor. You could also have some type of food poisoning that causes blood in stool. Please see your doctor as soon as you can. Be well.