Black smudge or black spot in goldfish is probably one of the most commonly seen goldfish health problems, and one that has many different potential causes.
Black spots or smudges are often indicative of disease or illness in the goldfish, and many goldfish will display blackening across part of the body or the fins and tails over a long period of time without the condition worsening greatly or appearing to have a massive impact upon the health of the goldfish.
However, black spots or smudges should not be considered normal and it is important to do what you can to tackle and treat the problem.