Just to clarify: In a low-voltage DC system, (like your car's electrical system and inside most of your computer), black would indeed be ground/common/neutral, and relatively harmless. However, in high-voltage AC systems (such as your house's wiring and appliances, and inside your computer's power supply), black is live (i.e. the "death wire") and white is neutral and green is ground. Thus our young fellow's confusion here.