The rapid expansion of the Internet of Things (IoT), particularly in critical infrastructures, necessitates strict security and privacy standards. Owing to data proliferation, Cyber-physical systems (CPS) rely on computing platforms for the provision of services and resources. The futuristic “Beyond 5G” (B5G) enabled critical IoT infrastructures cannot run on centralized systems due to their security vulnerabilities that compromise the basic Confidentiality-Integrity-Availability (CIA) triad. Blockchain technology (BCT) is emerging as a key enabler in addressing IoT’s security challenges, and it is compliant with the Fog-IoT architecture. The Ethereum platform has ushered an unprecedented development in BCT by facilitating application development. Blockchain (BC) connects the users’ chain identity to the transactions associated with their tokenized digital assets and confers the ability to audit the system. The history of canonical transactions is recorded in an immutable fashion facilitating data tracking and deterrence of data repudiation. A Consensus mechanism (CM) governs the state transitions and the node behavior in building trust relationships between various entities in the absence of a central authority. Through Smart Contracts(SCs), distributed and trustworthy access control can be achieved for IoT systems besides enhancing automation. We argue that BCT adoption is inevitable in securing futuristic B5G enabled IoT critical infrastructures for ensuring flexible and fine-grained access control, authentication, communication, and data security. Various challenges are associated with their adoption, such as the rising cost of Ethereum and constraints in the IoT environment. To facilitate BC solutions for IoT security, the functionality of BCT must be complemented with other technologies such as Machine Learning (ML), Edge Computing (EC), and InterPlanetary Filesystem(IPFS).