The increasing adoption of digital technologies in urban environments has led to the emergence of digital cities, where interconnected infrastructures rely on Internet of Things (IoT), artificial intelligence (AI), and distributed computing to enhance efficiency and sustainability. However, these advancements introduce significant cybersecurity challenges, including data breaches, identity theft, and infrastructure vulnerabilities. Blockchain technology offers a decentralized and transparent approach to data management and transaction validation, making it a valuable tool for improving security, privacy, and trust within digital cities. This paper examines the role of blockchain in securing and sustaining digital cities, with a focus on its integration with smart healthcare, intelligent transportation, agriculture, and supply-chain management. Additionally, it examines blockchain convergence with emerging technologies such as IoT, AI, federated learning, and zero-knowledge proofs (ZKPs). Furthermore, the paper analyzes key challenges, including scalability, energy consumption, interoperability, and regulatory compliance, and presents emerging solutions.