Yes as long as it’s not being accompanied by lots of pain or light headedness. This always hit around day 3/4 for me.
I’d 86 the stevia for fasting reasons and make sure you’re adding electrolytes to your water. Diarrhea is a very quick way to becoming dehydrated which feels like absolute garbage on a fast.
As a previous poster commented any black stool is concerning for digested blood unless you are on iron tablets. If they really are black I think you need to break your fast and see your doctor ASAP with a stool sample so it can be tested.