The concept of virtual power plant (VPP) has evolved to enhance the flexibility and reliability of power systems by integrating distributed energy re-sources (DERs). However, traditional VPP transaction models suffer from trust issues, such as default deliveries and payment arrears, which under-mine the welfare of trading parties and threaten systemic stability. To address these challenges, this paper proposes a blockchain-based trusted trading mechanism tailored for VPPs. The proposed mechanism is structured around a consortium blockchain with a hierarchical architecture, featuring one primary chain and multiple secondary chains. A reputation score-based Proof of Contribution (PoC) consensus algorithm is introduced to improve transaction efficiency and system security. The algorithm leverages historical trading behavior to dynamically adjust mining difficulty, incentivizing honest behavior among participants. Simulation results demonstrate that the PoC consensus algorithm significantly enhances transaction processing efficiency while providing robust defenses against malicious activities, paving the way for a secure and efficient VPP trading environment.