The blackened areas are where the cells have been damaged by the disease. This will sometimes clear after a while, as the cells repair themselves.
It can have a slight resemblance to amm. burn, but this is more under the scales as opposed to topical..
Myco is usually slow progressing, but as you read, it can act fast on weakened, or stressed fish.
As I'm sure you have also read, there is no known cure.
If the fish are kept in pristine water conditions, and fed a varied diet including fresh veggie matter, they can live for quite some time. I had bows infected with Myco live for 10 yrs. Obviously I took them out of my breeding rotation, and didn't add any fish to that tank. Don't use anything that is used in the infected tank on any other tank. If you have any open sores on your hands or arms don't put them in the tank. On a rare occasion humans can contract the disease. It can lead to some nasty sores. I dealt with it for years while breeding Rainbows, and never had any personal issues. You can still enjoy your fish for years with just a little perseverance towards clean water, and a healthy varied diet.
