In this work we investigated the impact of Nafion as a percolating agent of aromatic copolymer ionic channels. Membranes based on a blend of Nafion and copoly(arylene ether sulfone)s bearing perfluorosulfonic acid pendant chains (InX/Y) were prepared by casting. We demonstrated that, despite the partial immiscibility of flexible random Nafion and rigid aromatic ionomers, high-performance mechanically robust blend membranes could be obtained, exhibiting higher proton conductivities than those of the corresponding pristine ionomers. Thermal annealing reduced the water uptake while the proton conductivity was further enhanced, especially at low relative humidity. The best-performing blends were obtained with the annealed membranes containing 20 wt % Nafion; e.g., the proton conductivity reaches 3 times that of pristine Nafion cast in the same condition. These properties were correlated to the microstructure of the blend membranes, which was investigated by small-angle neutron scattering as a function of the blending ratio and the block lengths. The two ionomers were found to form phase-separated nanodomains, a morphology that is homogenized via applying thermal annealing. Nafion molecules are incorporated into the ion conducting phase of the aromatic ionomer, where they serve to better connect ionic channels, therefore providing facilitated pathways for a much more efficient long-range charge transfer.