The conductor has a clear insulator and around that there is a thin layer of black, electrically conductive plastic. Then there is the shield and finally the PVC jacket that makes up the rest of the cable. The black plastic layer must be stripped back well away from the conductor or it can short to it or at the very least cause a high capacitance that kills a lot of high frequencies.
The conductive plastic in conjunction with the relatively loose lap screen creates a 100% screen and helps stop tribolectric noise (caused by the cable moving).
