The continuous evolution from subsurface leakage to surface accumulation, delayed ignition and post-ignition consequences of buried hydrogen-blended natural gas (HBNG) pipeline leakage remains insufficiently understood, particularly under full-scale conditions. This study investigates the combined effects of hydrogen blending ratio (HBR) and leakage pressure on the release behavior, soil transport, enclosure stratification, and delayed ignition consequences of buried HBNG leakage. Thirty-two full-scale tests were conducted using a DN20 pipeline buried at a depth of 0.8 m with a 5 mm top leak orifice and a sealed 3.375 m3 cube enclosure, covering 10%–30% HBR and 0.13–0.73 MPa absolute leakage pressure. Mass flow rate increases linearly with leakage pressure, while the mass flow coefficient decreases from 15.71 to 12.19 g·s⁻¹·MPa⁻¹ as HBR increases. Vertical transport causes concentration stratification. Hydrogen migrates preferentially through soil and disperses rapidly in the enclosure, whereas methane became relatively enriched near the ground surface and top region. After leakage stops, pre-ignition concentration changes are evolved mainly by diffusion. Flame speed increases with HBR and reaches 2.42 m/s at 30% HBR. This study presents comprehensive experimental evidence for the coupled evolution of buried HBNG leakage, covering subsurface transport, enclosure stratification, and delayed-ignition dynamics. The results quantitatively reveal the mechanisms of component separation and vertical transport under various leakage pressures and HBRs. These findings provide a direct experimental benchmark for hazard assessment and CFD validation of HBNG pipeline systems.