Since the introduction of Bitcoin in 2008, blockchain systems have evolved immensely in terms of performance and usability. There is a massive focus on building enterprise blockchain solutions, with providers such as IBM and Microsoft already providing Blockchain-as-a Service (BaaS). To facilitate the adoption of blockchain technologies across various business verticals, we argue that middleware plays an integral role in accelerating the development of automated business processes (i.e., smart contracts). We argue that decentralized messaging is a key requirement of many distributed applications and should be provided as a reusable blockchain middleware. Our system, called HyperPubSub, provides decentralized publish/subscribe messaging for a multi-federated, permissioned, environment. HyperPubSub provides secure and privacy-preserving messaging, which is audited using blockchains for validation and monetization purposes. We demonstrate our implementation using Kafka and Hyperledger.