For large, intricate, and multi-layered networks like that of Industrial IoT, an individual instance of intrusion detection system cannot efficiently work against advanced attack strategies. The reason is that it would not be aware of the overall context, environment, and relevant incidents in other networks. This necessitates a collaborative intrusion detection system that allows multiple intrusion detection systems to communicate with each other and share information on emerging cyber-attack incidents. Thus, immunizing themselves and preventing the attack from escalating. However, the main challenge here is to manage the trust among the peers, where an insider attacker may input false attack signatures to the network, thus degrading the performance. Hence, we propose a blockchain-based trustfree collaborative intrusion detection system, in which threat alert messages will only be propagated in the network after network consensus.