Blue bonds are gaining global traction as innovative financial instruments to tackle marine sustainability, yet their yield spreads compared to conventional bonds remain unexplored. Based on the growing interest in sustainable investments and the concept of the greenium, this study introduces and searches for a bluenium, the analogous premium for blue bonds. Hence, the purpose of this research is to investigate whether blue bonds exhibit a lower yield at issuance compared to conventional bonds. This examination is intended to contribute to the literature on impact investment risk and return, particularly in the context of marine sustainability, providing valuable insights for investors, issuers, researchers, and policymakers. The study employs the propensity score matching (PSM) method to ensure robust comparative analysis between blue bonds and comparable conventional bonds. The empirical analysis identifies a yield spread of 47 basis points (bps) favouring higher yields for blue bonds, though these results lack statistical significance. Hence, there is no significant evidence of lower yields for blue bonds compared to conventional bonds. The insignificant results could stem from the relatively small sample size, reflecting the fact that blue bonds are in their early stage, suggesting that they may require further development, similar to what green bonds experienced. Future research should consider larger samples and additional variables to enhance the robustness and applicability of the findings. This study informs stakeholders of the complexities and development potential of the blue bond market.