Some discolouration of copper is normal, as long as it’s green and inside the pipe. Green colouration on the outside may be a sign of leakage. Black colouration anywhere on the pipe is not normal. It’s often a sign that something is going wrong, and your plumbing is being corroded. The culprit may be water with a pH that is too high or too low. This is more common in well water systems, as city water should be treated to normalize the pH.