This chapter examines the legal architecture of blue bonds as a novel instrument of impact investing designed to finance sustainable marine and ocean-related projects. It situates blue bonds within the broader framework of green and sustainable finance, analysing their defining features, market evolution, and potential vulnerabilities to greenwashing practices. Particular attention is given to the contractual and regulatory challenges posed by the absence of a dedicated legal framework, including issues of transparency, traceability of proceeds, and enforcement of non-financial obligations. The chapter argues that while self-regulation and market standards—such as those developed by the International Capital Market Association—play a crucial role in fostering market credibility, they are insufficient to prevent opportunistic behaviour and ensure investor protection. Against this background, the European Union’s Green Bond Regulation and Taxonomy Regulation are explored as key instruments capable of providing a robust normative foundation for blue bond issuances. The analysis concludes that developing clear legal standards and effective enforcement mechanisms is essential to reducing the risk of “bluewashing”, enhancing investor confidence, and securing long-term private capital for the conservation and sustainable use of ocean and marine resources.