Yes, the black noxious sludge of a fasting BM is normal. In regards to the weight, I would suggest ditching the scale once and for all. Sure, it is nice to see ”progress” but to add worry and stress to your mind during a fast because of odd rises and drops is completely unhelpful to the fasting mind, creating unease where none is necessary. To answer the question, weight fluctuations are normal and part of the physiological process of a fast. Has much to do with water.