Corrosion on copper pipe: Is this a concern

You may notice a rotten egg odor from the water from time to time if the sulfur content is high enough. Bacteria in the water convert sulfur compounds into hydrogen sulfide which reacts with the copper to form a coating of copper sulfide.

There are no set limits on how much sulfur can be in the water. It's naturally occurring and not considered to be a primary water contaminant.

If you are having problems with slime, stained clothes or rotten egg odor, chlorination can reduce the bacterial action that produces hydrogen sulfide. Chlorination will cause the sulfur to precipitate out of the water which you will now have to filter to remove the particles.

Excessive sulfur content in water has a laxative effect, probably the only reason to worry about it. Lower content is basically an added nutrient. Hydrogen sulfide gas in quantity is a poison, but since it's smelly and you haven't mentioned it, the quantity is likely a non-issue.

As to the pipes, it sounds like your problem is a stable black copper sulfide tarnish that has low solubility in water (unlike copper sulfate which readily dissolves and is toxic). New copper will quickly acquire the same coating from the hydrogen sulfide (which will also tarnish silver).

I cannot answer whether it needs replacing, but I can tell you why this happens.

It happens because it gets cold and water condenses on it.

Whenever you flush your toilet, cold water runs through and cools it off. Then the hot, humid air around it condenses water on it, making it moist. Because this happens so often, it will be moist a lot of the time, and thus corrodes.

Why this doesn't happen other places: The toilet is the one thing that uses most cold water. Your shower and sink uses more warm water, and not as much, and thus aren't as exposed to this phenomenon.

Unless it leaks I think you can leave it on.