According to my CB500F manual the black connects to the black/white wire when the 'Kill' switch is in the 'on' position.
But the 500 version switchgear is a bit unusual to the other fours in that the left of 4 positions is 'Off', then 'Low beam', then 'Pilot', then 'High'. So, the black wire is used in the last three positions to provide power to the lights. It may well be that the black contacter inside of the switch has corroded, which is easily checked with a multimeter. Connect one end of the resistance meter set on up to 100 ohms or so, and buzz from the black wire of the main lead into the switchgear, then with the light switch in the 'L' position, check continuity to the brown/white wire and the white wire. Then move the switch to the 'P' position, and buzz from the black to the brown/white wire only. This is the 'Pilot'. Then move the switch to the 'H' position and buzz from the black wire to the blue and the brown/white wires. If non of these gives any readings , but the black and black/white wires buzz through ok with the kill switch in the 'on' position, then it's definitely the black wire contact in the switchgear. This is easily disassembled, if you are careful, and clean all of the contacts...just don't lose the small spring or the ball bearing.
Later switchgear had the low/high switching on the left side switchgear, and only 'Off', 'Pilot' and 'On' positions on the right side.
