Distributed database systems provide scalability, availability, geographic distribution, and workload flexibility, but their decentralized operational model introduces difficult security and trust-management problems. Conventional controls such as centralized access-control servers, database logs, perimeter defenses, and replication mechanisms can protect individual components, yet they may not provide a shared, tamper-evident history of sensitive transactions across mutually distrustful administrative domains. This article proposes a blockchain-based security framework for distributed database systems that combines permissioned blockchain, cryptographic hashing, identity and access management, smart-contract policy enforcement, database transaction monitoring, and immutable audit trails. The framework keeps operational data in the distributed database while placing selected hashes, metadata, policy decisions, and audit commitments on a blockchain. This separation is intended to reduce blockchain storage overhead while preserving verifiability. A layered architecture is presented covering application, identity, blockchain, integrity/audit, and database layers. The article also describes a transaction workflow, threat model, security controls, implementation considerations, performance metrics, applications, challenges, and future research directions. A conceptual evaluation methodology is provided for measuring latency, throughput, integrity-detection capability, storage overhead, availability, and administrative cost. The proposed approach is positioned as a complementary security architecture rather than a replacement for encryption, database access controls, secure development practices, or conventional monitoring.