Coastal blackouts as anti-submarine defense

Who devised coastal blackouts, when were they used, and why were they effective?

Blackouts prevented shore lights from silhouetting ships for U-boats and other attackers; the American coast in World War II illustrates their value, but no originator is identified.

A ship between a submarine and a lit coastline could be silhouetted even when the ship itself showed no lights. The illuminated shore outlined the target for an attacker at sea. Coastal blackouts removed that backdrop and made merchant ships harder to find at night. Delayed blackouts after the United States entered World War II helped U-boats locate and sink ships along the American coast. No particular originator or first use of the measure is established.