The black on the fins and head is normal colouration as mentioned by Gary. The black near its butt could be colouration or chemical burns, which appear as black on fish. However, the fact it has black pigment coming in on the head and tail would suggest the black on the body is just colouration too.
Chemical burns can be from ammonia, nitrite, really high nitrate, chlorine, chloramine, or overdosing medications or plant fertilisers. As long as you don't have any ammonia or nitrite in the water and the nitrate is less than 20ppm, and you dechlorinate new water before adding it to the tank, and you don't overdose medications or plant fertilisers, you are unlikely to see chemical burns on fish.
On a side note, the water in the picture looks a bit milky cloudy and this is usually caused by uneaten food rotting in the tank and a new filter. Do a 75% water change and gravel clean the substrate every day until the water looks clear and no longer has a milky cloudy appearance to it.
Make sure any new water is free of chlorine/ chloramine before it's added to the aquarium.
If it's a newly set up aquarium, you should check the ammonia, nitrite, nitrate and pH every few days to make sure there water is free of these things.
