Highly efficient light-emitting diodes (LEDs) based on metal halide perovskites with green, red, and near-infrared electro-luminescence have been widely demonstrated. However, the development of their blue counterparts is still hampered due to the difficult deposition of efficient and spectral-stable blue-emitting active layers. Here, we report a facile and general approach that uses a moisture treatment in combination with the precursor stoichiometry engineering for the fabrication of efficient and color stable blue-emitting perovskite films. We find that, with a short-term moisture exposure, light emission from Ruddlesden Popper lead bromide-based perovskite films exhibit a continuous blue-shift from 512 to 475 nm through incorporating excess CsBr in the precursors. In addition, we observe that the formed Cs4PbBr6 phase under CsBr-rich condition is favorable to stabilize the blue emission of the resulting films. The corresponding blue-emitting perovskite films exhibit a photoluminescence quantum efficiency of over 20%, delivering sky-blue perovskite LEDs with no change in the light emission even under high voltage. Our strategy provides an alternative way for realizing efficient and spectrally stable active layers for the further development of blue-emitting perovskite LEDs.