Intelligent Transportation System (ITS) with the internet of things (IoT) plays an integral role in smart city developments and enables substantial developments in modern human lifestyles. With the emergence of Fifth-Generation (5G) communication technologies, high-speed communications are enabled among multiple internet-connected devices. However, security, reliability, and scalability are significant factors that affect the communication performance of ITS. The conventional security models are mostly centralized and unsuitable for distributed low-powered IoT-enabled 5G ITS. The new-age distributed ledger technology blockchain can improve the security and reliability of ITS services. Therefore, this paper investigates a blockchain-based security mechanism, Blockchain-based Secure IoT Communication (BSIC), that protects the 5G-ITS from potential security threats. The BSIC utilizes a consortium blockchain model with Improved Proof of Reputation (IPoR) to achieve its objectives. It handles the resource limitation issues of IoT by integrating Vehicular Edge Computing (VEC) services. Further, the BSIC design includes two main components: reputation computation strategy and the IPoR mining process. The proposed model successfully builds a secure IoT communication system by integrating multi-criteria factors in subjective logic-based reputation estimation. It selects the miners by adjusting the consensus pool size according to network density and reputation and improves the consensus efficiency with minimum delay. Moreover, the experimental evaluations are carried out to analyze the efficiency of BSIC using performance metrics such as attack detection rate, consensus delay, and reputation estimation accuracy.