The black patching is bruising or chemical burns and is normally found on fish that have been exposed to high levels of medications or other chemicals like chlorine or chloramine.
It's possible a bacterial infection is developing in the sores but that will normally appears as a red area.
In one image (89775), the fish appears to be missing the scales in the black area. I can't tell anything from the other 2 pictures. If the scales are falling off they should regrow assuming an infection didn't cause them to drop off.
I would monitor the fish and try to get some better images and see how things go.
You can try doing a 75% water change and gravel clean the substrate each day for a couple of weeks. That usually helps fish. Clean the filter if it hasn't been done in the last 2 weeks. Wipe the inside of the glass down whenever you do a water change.
You can try adding a fish vitamin supplement to its diet. If you can't get any fish vitamins then look for a reptile or bird vitamin supplement. If you use reptile or bird supplements add it to the food once a day for 6 days and do not use it on the 7th day. If you use a fish vitamin supplement you can use it twice a day for a few weeks and then follow the directions on the packet.
Try to get a dry powder vitamin supplement rather than a liquid one, and check the expiry date before you buy it.
You can give vitamins to the other fish too.
If you have carrot or any orange or blue fruit, you can cut them up into little pieces and feed the fish that. You can also juice orange and purple fruits and veges and soak the food in the juice for a minute before feeding. Use a frozen (but defrosted) food for this.