I'm pretty new to the sourdough game and will admit that I have very little knowledge of the science behind it (hydration, etc.). I have so far tried two sourdough starters to make two simple sourdough boules which have both failed.   The first was a Paul Hollywood starter and bread recipe. My starter seemed active as described in the book (it would double in size and recede each day when feeding) yet when it came to make the bread my dough would be semi-shapable but would end up sticking to a well-floured (I really mean well-floured...) proving basket and ended up ruining any proving that had occurred rendering it useless.  The second starter I made following this starter recipe from Youtube and this bread recipe from the same person. I decided to go along the video route as I thought I could better see how my dough should look and feel at particular stages. The only difference was that I initially kneaded mine in the KitchenAid Mixer with dough hook to bring it together. For the most part my dough was sticky and wet as expected but didn't quite come together to form the cohesive shapable dough that he seemed to get. I tried adding a bit more flour but it seemed as if I would need to add a lot more to get to the dough that he had. I was able to shape the dough into a ball to place into a well-floured proving basket but again it simply stuck to it when it was time to place it into the oven and lost all air structure from the proving.   Any ideas what I'm doing wrong? 
Those dark spot could be either due to physical damage.  For instance like banana or potato, if you hit them too hard, they will "bruise". That could be one of the cause. The other cause you see dark spot in potato means it is starting to show sign of rotting. Usually potato that are about to spoil tend to have this dark spot. If you leave this potato uncooked with the dark spot for another few days, you would realize root will start growing and by then the potato is not recommended to be eaten. This happens in both normal, purple or sweet potato. So either you can choose to cut off the dark part away from the potato before eating or you can throw it away ( if you're very particular about things like that.   P.S : If you always pick potato like that, i recommend you giving suggestions to the shop where you are buying them from. Fresh potato will look like the one on the right. To be honest, some shop may buy "poorer quality of potato" at a cheaper price and pass it off as "fresh" potato, so do be careful about that too. Just my two cents, not generalizing shops that sell such potatos. 