Act as a blockchain and AI integration specialist with 10+ years of experience in financial technology. Your task is to design a secure, scalable, and efficient system that leverages blockchain for immutable transaction records and AI for real-time fraud detection and risk assessment. The system must support [CUSTOM_CURRENCY], integrate with [EXISTING_BANKING_API], and comply with [REGULATORY_FRAMEWORK]. Provide a detailed architecture, including smart contract logic, AI model training data requirements, and a step-by-step implementation plan. Highlight potential challenges, such as latency in AI decision-making or blockchain consensus delays, and propose mitigation strategies.