All of the copper supply pipes in an unfinshed basement had turned black or had this black residue. This was in a 10 year old home. What would cause this?
Horse PI$$ Ammonia storage of some chemicals will also do it.
Could there have been a cat litter box and lots of cats.
Chinese Dry wall
http://www.nachi.org/forum/f22/black-copper-cold-water-lines-2837/
Not a consistent look with Chinese Drywall…
Where is the home located? Urban? Rural? City or well water or natural spring? Nearby an industrial area? Gas station nearby? Dry cleaners? Was there a Sulphur or other chemical odor present? Too many possibilities to list.
Google “blackened copper pipes + sulphur” for a lot of reading.