You should definitely be skeptical if the products you are taking are pitch black like tar and smell worse than a toxic waste dump going in.
If this is just an elaborate ruse, then I'm sure the creators of such products spent millions doing research to find a combination of substances that would undergo such a radical chemical and color change during digestion.
Since innocent until proven guilty, can the accusers produce a list of herbs that will putrify at warp speed, create a horrifying smell and cause gray oil to pour out and float?