Flexible resources in built environments are seen as a low-cost opportunity for delivering grid management services. Consequently, the centralised aggregator model, where the aggregator is used to bundle demand flexibility from flexible resources and deliver it to flexibility customers such as Distributed/Transmission System Operator (DSO/TSO) in flexibility markets, has been adopted. However, the aggregator role introduces various security and trust challenges. In this work, we propose a blockchain-based flexibility trading framework dubbed PlexiChain to address the security and trust challenges the aggregator poses in the centralised aggregator model. The security evaluations performed using a real-world dataset show that PlexiChain is robust against known security attacks, such as MadIoT and False Data Injection attacks. Additionally, the performance evaluations show that PlexiChain has lower computation and communication costs than other blockchain-based applications in resource-constrained environments.