Traditionally, electricity is traded in some markets, e.g., day-ahead spot, while flexibility is traded in other (regulating power, ancillary services) markets. With the advent of distributed electricity production, energy communities and flexible loads like heat pumps and Electric Vehicles (EVs), there is an unmet need to consider energy and flexibility as two sides of the same coin and trade them jointly instead of separately. This paper describes a solution for joint trading of (electrical) energy and flexibility, e.g., within energy communities. The solution is based on the so-called FlexOffer concept, which models energy needs and their associated flexibilities in one joint data object. The paper explains the actors and procedures for trading and market clearing. It also describes an implementation based on the IBM HyperLedger Fabric blockchain platform and experimental results from a pilot in the FEVER EU Horizon 2020 project.