The black substance is most likely to be copper oxide, which is formed when copper comes into contact with Oxygen in the air. On some of the poles there is only the fat black cable so I speculate that they are fibre joins. What confuses me is that in some cases they are only two poles apart. I. Have you ever noticed a Black line on a wire while working on a DIY electrical project or simply plugged in an appliance? That thin, dark stripe can be intriguing, but also worrisome. If you're. At Palmer Electric, our certified electricians work with black wire every single day, in homes, in commercial buildings, in new construction, and in older properties where the wiring has been modified over the years.