Vehicular ad hoc networks (VANETs) have emerged as a critical solution for improving traffic flow, raising passenger comfort, and enhancing road safety; as a result, it facilitates easy connection between automobiles, central control systems, and infrastructure. However, despite their potential, VANETs face significant challenges in maintaining reliability and service quality due to its vulnerability to security threats posed by malicious nodes, which can disrupt communication, compromise data integrity, and lead to severe consequences like traffic congestion, accidents, or system failures. By addressing such challenges, this paper presents BDMV, a blockchain-based defense mechanism for VANET that includes a novel dynamic trust-based security model designed to accurately identify and mitigate black hole attacks by detecting malicious nodes. The proposed trust model is further strengthened by integrating blockchain technology, which enhances security through decentralized data validation and authentication within the blockchain framework. Through comprehensive experimentation, the model demonstrates significant performance gains, including a 29.64% reduction in total delay and average improvements of 36.83% in threat identification and 32.17% in throughput, surpassing current state-of-the-art approaches.