Blockchain technology has made significant success, but it is also vulnerable for cyberattacks. As a particular form of attacks, Black Bird 51% hash rate attack has not been studied in depth, and it deserves particular attention. It is important to analyze its core construction, origin, and perniciousness. Black Bird Attack (BBA) could jeopardize a decentralized system more than centralized one, because centralized system has higher priority to control the dataflow than its users, whereas decentralized systems are vulnerable to attack, since everyone has almost the same privilege. Black Bird Attack can potentially destroy the fairness of blockchain networks. With this in mind, we have built a blockchain network simulation to examine this form of attacks, as presented in a detailed case study. This case study demonstrates Black Bird Attack is completely possible. Since Black Bird 51% hash rate attack is particularly dangerous in the post-quantum computing era, an intensive research agenda on this topic is in urgent need.