The reduction of climate change effects (mitigation) is now mainly driven by the limitation of CO2 emissions which is currently becoming increasingly necessary and urgent to achieve. In this context, within the past Horizon 2020 program, the Urban Europe Joint Programming Initiative aimed to promote the development and deployment of at least 100 Positive Energy Districts by 2025. The latter are flexible, energy-efficient urban areas or groups of buildings that produce zero net CO2 emissions and actively manage annual renewable energy surplus production at the local or regional level. As they are connected to the electric power system, Positive Energy Districts have the ability to offer flexibility services to the grid, such as frequency or voltage regulation, being part of a larger scheme as the Italian energy communities recently regulated by the Milleproroghe Decree. In this paper, a blockchain-based architecture is proposed with the aim of providing a system for the trusted certification of energy self-sustainability and the management of energy trading among different districts or the supply of energy services delivered from the district to the electricity grid, promoting their effective integration with the power system.