The black colour comes from carbon black added to the outer layer of the covering. Carbon black absorbs ultraviolet light and stops sunlight from breaking down the polymer, which is what lets an overhead conductor sit exposed to the sun for decades. Without it, the outer surface would gradually crack and chalk.
Only the outer layer carries the carbon black. APAR's covered conductors have three bonded layers over the conductor: a semiconducting screen, an inner layer of pure, carbon-free XLPE that provides the insulation, and an outer XLPE layer that is UV-resistant and anti-tracking. Keeping carbon out of the inner layer protects its electrical performance, while the outer layer handles sunlight, pollution and surface tracking. Construction details are on the covered conductor product page, and the role of the outer layer is explained in why a covered conductor needs an anti-tracking layer.