Some of my leaf that got too wet in the bottom of the plastic bag has turned black like that. You may have too much moisture. It shows 81% on your gauge, I would try to get that down to 60-70% and try to get the temperature up between 120-125F. I would especially worry about mold at that low temperature and high humidity. I normally ferment about one month. Also it may help to move the tobacco around in the bag to help keep temps and humidity more uniform through the tobacco since there is little to no air flow. Best of luck.