Working on my new house, I noticed the coax cable coming up out of the ground the center conductor is like black (instead of a shiny copper color). It could be the wire just being exposed to moisture that is causing the discoloration.
Since you replaced the leads with new wire, try cutting the old wire back a few inches and see if it looks the same or if its just at the very ends of the wire where it was exposed.
Since you replaced the leads with new wire, try cutting the old wire back a few inches and see if it looks the same or if its just at the very ends of the wire where it was exposed.