When hiring a blockchain developer, focus on their proficiency in key programming languages like Solidity, JavaScript, Python, or Go. Experience with popular blockchain platforms such as Ethereum, Binance Smart Chain, and Hyperledger is essential. You should also prioritize their ability to develop secure and efficient smart contracts. Additionally, knowledge of cryptography, distributed systems, and peer-to-peer networking will ensure they understand the underlying security and scalability concerns of blockchain. Strong problem-solving skills and experience with Web3 tools like IPFS or MetaMask are also beneficial.