The proliferation of collaborative organizational environments has exposed critical limitations in traditional risk-based access control systems, including multi-role user assessment complexity, local data dependency limitations, and heterogeneous risk metric standardization. This paper introduces COBRA-Z (Collaborative Blockchain Risk Assessment for Zero Trust Architecture), a novel framework that employs standardized log structures to capture user behavioral data and leverages blockchain smart contracts to securely orchestrate transparent and auditable risk assessment algorithms across organizations, enabling collaborative trust management in Zero Trust environments. We implemented and evaluated the framework using Hyperledger Fabric, demonstrating practical feasibility through comprehensive performance analysis.