Hydrogen-blended natural gas transportation is an important pathway for carbon reduction. However, hydrogen addition significantly enhances the deflagration intensity of methane, imposing more stringent requirements on the reliability of metal wire mesh flame arresters, which are key pipeline safety components. In a horizontal closed duct, this study systematically investigated the coupled effects of mesh density (10, 30, 60, and 80 mesh), hydrogen fraction (0, 10%, 20%, and 30%), and wire mesh integrity on the pressure–flame propagation characteristics and heat loss behavior. The results show that increasing mesh density reduces the maximum explosion pressure by 38%-45%, while the maximum rate of pressure rise exhibits an initial increase followed by a decrease. Correspondingly, under the influence of mesh density, the pressure–time curves evolve from a single-peak synchronous response to the emergence of double peaks and finally to distinct double-peak separation. At low mesh densities, once flame arrest fails, the flame–mesh interaction generates turbulence that accelerates flame propagation, and the flame is highly likely to trigger reverse propagation. Wire mesh that remains structurally intact during the explosion can effectively reduce the explosion pressure even when quenching fails. However, once structural damage occurs, the suppression capability is completely lost, and the damaged mesh instead acts as an obstacle, inducing intense turbulence and drastically accelerating flame propagation. Compared with the no-mesh condition, the heat loss at PPI = 80 increased by 52%-84% across the tested hydrogen fractions. Furthermore, it is revealed that flame quenching is governed by the competition among three effects, namely mesh quenching, stagnation–penetration, and turbulence enhancement. This research provides a theoretical basis for the reliability design and safety assessment of industrial flame arresters under hydrogen-blended natural gas conditions.