I have not had Bacterial dermatitis, but this sounds very similar to Photosynthization (sp) or an allergic reaction to the sun. Over the years I have had two of these . Eventually any white ( or lighter colored areas) will shed the hair and skin. The darker areas will be covered with scars and really look bad. Four years ago, I found one of my donor cows in the pasture with this condition. Her nose looked like old shoe leather, and was extremely sore. Her calf was doing fine, however her udder looked like her nose, so the calf was not getting much milk. I took her to the vet and this was what was diagnosed. I had to put her and her calf in the barn, in total darkness for several weeks and keep her on antihistamines for most of this time. I had to cover all the windows so that no light could get in. I felt so sorry for both her and her calf, but they both survived and got better. The heifer calf rebounded quickly and was good enough by fall to be selected for the Agribition sale, and sold for $4000. The cow was held open and was flushed successfully through the fall and winter. Once her skin shed off and new skin grew, she got better fast. Once she went back out on grass,after she was better, I swore she gained 20 lbs a day. I have never seen a cow change so quickly. The only other case I have seen was a neighbour had over 20 cows come down with this several years ago. The vet suggested it was triggered by an allergic reaction to something in the feed.
Not sure if your calf is like this or not, but the skin condition does sound similar.