Blockchain technologies enable new forms of data sharing in platforms. This raises questions around how they are jointly developed and managed in blockchain consortia and what role public agencies play in those efforts. Based on an analysis of prior work on data sharing in public-private partnerships and other blockchain projects, we analyze the case of the Cardossier. The Cardossier project and (later) association develops a platform to link the public and private actors in the Swiss car ecosystem. The participating car registration authority has the roles of an actor in interorganizational processes, supplier of data, source of trust, guarantor of data quality, user of data, and incentive for making goods public. We conclude that the public agencies have a very important role in blockchain consortia and propose that they should use this role actively as part of their efforts to create public value.