The two major roadblocks for state of the art Internet of Things (IoT) infrastructure like smart buildings, smart cities, etc. are lack of trust between various entities of system and single point of failure which is a vulnerability causing extreme damage to the whole system. This paper proposes a blockchain based IoT security solution where, trust is established through the immutable and decentralized nature of blockchain. The distributed nature of blockchain makes the system more robust and immune to single point of failure. We propose a mechanism to establish continuous security in the system by evaluating legitimate presence of user in valid IoT-Zone continuously without user intervention. Every userinteraction in an IoT environment is stored in blockchain as a transaction and series of these transactions represent a user’s IoT-trail. A unique digital crypto-token is required for a user interaction to be legitimate. This token is used as an access control mechanism to prevent any unauthorized access to the system. Tokens are pre-generated using a prediction model based on user’s IoT-trail in the blockchain. By using blockchain as an underlying framework in IoT environment and through the method of continuous security, we made the system more secure, robust and interoperable.