It is the chemical reaction between the different metals that cause the electrolyzes. I have found this out years ago when I was contracting and my plumber had to re plumb an entire house because in the crawlspace there was a galvanized nail touching it.
I find it very interesting all the different opinions on this subject, the best one Was the meth lab theory. Sulfur could turn a pipe color but there would be sulfur in the hot and cold water not just the cold water and it does not turn the out side of the pipe black.
Check and see if the electrical panel is bonded to the plumbing, if it is check the bonding clamps copper pipe should be bonded with brass clamps and galvanized pipe should be bonded with galvanized clamps. When these bonding clamps are not correct I always call them out for this reason.