Achieving efficient blue electroluminescence (EL) remains the fundamental challenge that impedes perovskite light-emitting diodes (PeLEDs) towards commercial applications. The bottleneck accounting for the inefficient blue PeLEDs is broadly attributed to the poor-emissive blue perovskite emitters based on either mixed halide engineering or reduced-dimensional strategy. Herein, we report the high-performing sky-blue PeLEDs (490 nm) with the maximum EQE exceeding 15% by incorporating a molecular modifier, namely 4,4'-Difluorophenone, for significantly suppressing the non-radiative recombination and tuning of the low-dimensional phase distribution of quasi-2D blue perovskites, which represents a remarkable paradigm for developing the new generation of blue lighting sources.