I have two female mice, about 6 months old. Recently the black mouse
started getting a growing single black sore
on his back that looked like a scab. It was about centimeter wide. The hair on that spot had fallen out. Since then he now has two other smaller spots, separate to the original one, but close by. There is no redness/bleeding or inflammation, just a hard scabbed area. The other mouse is perfectly fine, and both mice show no signs of mites, but I haven't had a good look yet.
Here's a picture (copy and paste the link into a new tab/window); it was pretty hard to get her to sit still:
(please excuse my dirty finger nails)