This study examines the role of cryptographic hashing as a foundational security primitive in blockchain-based digital transaction systems, with particular emphasis on integrity, transparency, auditability, and long-term cryptographic resilience. The research evaluates how hash-linked block structures, Merkle tree constructions, and decentralised consensus mechanisms enhance transaction immutability and resistance to unauthorised data modification when compared with traditional centralised architectures. Special attention is given to the implications of emerging quantum computing threats on classical public-key cryptographic schemes and the suitability of hash-based and postquantum signature approaches, such as XMSS and SPHINCS+, for future-proof blockchain security. Using analytical comparison and conceptual system modelling, the study demonstrates that hashing-centred blockchain architectures significantly improve evidential integrity, traceability, and non-repudiation, making them particularly valuable for forensic accounting and auditing applications. The findings confirm that distributed hash verification eliminates single points of failure, strengthens audit trails, and supports transparent, tamper-evident financial recordkeeping. While hash-based and postquantum mechanisms introduce additional computational and storage overhead, their resilience against projected quantum adversaries provides substantial long-term security benefits. Above all, the study concludes that the integration of cryptographic hashing, Merkle tree structures, and postquantum–aware signature schemes offers a robust and sustainable framework for secure digital transactions. These mechanisms not only enhance current blockchain performance but also position blockchain systems as reliable infrastructures for future digital economies, regulatory compliance, and forensic assurance in increasingly complex and adversarial computational environments