The Blockbuster was a specific type of bomb used by the British RAF during World War II. These were large, high-capacity bombs (initially 8,000 lbs, later 12,000 and 22,000 lbs) used for area bombing. The name 'Blockbuster' comes from their ability to destroy an entire city block. While atomic bombs were used in WWII, the question specifically asks about the named bomb 'Blockbuster'.