In this paper, we provide a concise review on the use of blockchain in space-air-ground integrated networks (SAGIN). Our study is guided by several research questions. First, we identify the objectives of the blockchain-based solutions for SAGIN. Second, we investigate how the blockchain technology is used to achieve the objectives. Third, we evaluate the maturity levels of the proposed blockchain-based solutions. Our findings show that blockchain has been used in the areas of dynamic spectrum management, crowd-sensing, traffic offloading, computation offloading, geofencing for unmanned air vehicles, data sharing, authentication of various SAGIN-connected entities, security of air traffic management, data security, and trustworthiness of the network. All studies proposed some forms of custom blockchain instead of using an existing public blockchain platforms to achieve their intended objectives. Furthermore, most studies chose to use some traditional distributed consensus algorithms and other non-mainstream algorithms for the proposed blockchain solutions. Except for a couple of studies, in which Hyperledger was used as the basis for customization, none of the studies validated the proposed solution experimentally. In addition to the research questions, we highlight pervasive misconceptions and misunderstandings regarding the blockchain technology and provide a guideline on how to properly develop blockchain-based solutions for SAGINs.