The increasing demand for trusted and verifiable digital microcredentials in education and professional development has created new challenges related to data integrity, decentralization, and service scalability. Although blockchain technology has been widely proposed as a potential solution, existing approaches often lack operational rigor, security formalization, and alignment with service-oriented computing paradigms. This paper introduces Skillchain, a blockchain-based microcredential management platform implemented on Ethereum, designed as a secure, decentralized trust service. We present a full-stack architecture that incorporates a hybrid network model and a Proof-of-Authority consensus mechanism, enabling efficient issuance, revocation, and validation of digital microcredentials. A key contribution of this work is the integration of smart contract–based access control and on-chain governance, providing trusted organizations with structured authorization workflows. The proposed platform is evaluated through a comprehensive set of performance and security experiments, which demonstrate scalability to thousands of concurrent transactions and resilience against misconfigured node vulnerabilities. A formal threat model is developed, and countermeasures are implemented at the protocol and smart contract levels. Comparative analysis with centralized and federated models is provided, highlighting trade-offs in latency, transparency, and trust distribution. This study advances the state-of-the-art in blockchain-enabled service computing by offering a reproducible, extensible service architecture for secure credential management. The findings highlight the platform's potential for integration with interoperable credential ecosystems and contribute to advancing decentralized service computing for trusted digital credentials.