Blending hydrogen into existing offshore pipelines provides a cost-efficient solution for future offshore hydrogen transportation. However, hydrogen blending can increase leakage possibilities, and its effects on resulting bubble plume dispersion remain unclear. This paper systematically investigates the impacts of hydrogen blending ratios on underwater bubble plume dispersion, rupture behaviours and potential safety risks. The results show higher leakage rates promote a dispersed-to-continuous bubble plume transition. Increasing the blending ratio delays this transition. With no blending, plume rupture occurrence probability drops to 0 when leakage pressure is 0.30 MPa, but with 20 % blending this shifting to 0.40 MPa. Rupture probability is still at 46 % even at 0.50 MPa with 100 % blending. The correlation between dimensionless plume penetration length and Froude number under varying blending ratios is proposed with R2 = 0.93. Increased leakage rate and blending ratio shorten rising time and demonstrate substantial attenuation before 0.20 MPa, increasing propagation velocity linearly. The maximum fountain height varies greatly under different blending conditions, while the steady fountain height is comparable. Increasing blending ratio persistently diminishes the dimensionless heat release rate and consequently lowers the dimensionless flame height. These findings provide theoretical support for safe offshore hydrogen transport and risk management.