If you find black coloured cables entering the consumer unit, this is an immediate sign that the wiring needs replacing, it has been installed in the 50s and inadequate for today’s use. Modern electrical cables are manufactured in white thermo plastic sheathing and are insulated by a coating of TPS, this cabling also has red for (live), black (neutral) and most importantly green/yellow for earthing. – the old black cables were insulated with rubber, which perishes and decays with age. You will also find there will be no earth present with this type of PVR cabling.