The fast evolution of intelligent systems and distributed networks, along with the Internet of Things across domains, leads to the need to authenticate a massive number of nodes from different domains to share their data and computation in ultradense IoT networks. Although different centralized and decentralized authentication approaches based on conventional cryptographic and blockchain-based mechanisms have been proposed, few of them consider the requirements and the trade-off between security, efficiency, and the vertically isolated IoT network problem for cross-domain communication. This work presents a robust and efficient massive authentication approach suitable for Cross-domain IoT based on hybrid blockchain and lightweight group authentication. Our approach not only solves the interoperability and scalability problem of cross-domain authentication for heterogeneous IoT devices through a decentralized mechanism but also proposes an efficient authentication approach suitable for a massive number of IoT devices with different resource constraints. In order to validate the performance of our proposed framework, we implement it on three different resource-constrained IoT devices and analyze their efficiency for real-world IoT networks.