Note that in the description in the link you gave that other bacterium are often isolated from infected fish. What this means, based on my experience, is that bacterial septicaemia can be caused by a wide variety of bacteria and one shouldn't jump to conclusion that it's Mycobacterium marinum. You can't really tell from the external lesions, such a black patch/spot. If your fish dies and you decide to cut it open and you see lots of cottage cheese-like lumps in the visceral organs (aka granulomas) - that's a different story! Even then, however, granulomas don't positively mean M. marinum is involved as many other bacteria can cause these types of lesions. Note that other things can cause black spots, in particular localised skin reactions (aka melanization) to parasite infections. Looking at your pictures, however, leads me to suspect your fish might have a bacterial infection of some sort. Note the haemorrhaged areas on the caudal peduncle and the area where the soft dorsal rays insert. You might look for other signs of bacterial infection (swelling, pop-eye, and additional lesions with haemorrhaging, and in particular small red spots, listlessness, to name a few key signs). If this is the case you should consider an antibiotic treatment, preferable using an orally administered preparation (eg: Seachem Focus + any of Sulfathiazole/Kanaplex/Neoplex or Aquarium Products Gel-Tek Ulta Cure BX). If, ultimately, your fish is infected with M. marinum, which is a possibility, don't get your hopes up of successfully treating it. Infections from this bacterium are very difficult to treat.
Hope this helps - good luck with it.
