Despite the excellent performance of hybrid organic-inorganic halide perovskites-based red and green light-emitting diodes (LEDs) with external quantum efficiencies (EQEs) ~ 25% and luminescence (L) ~ 10⁴-10⁵ cd/m²,^{[1, 2]} desired pure blue-emitters and corresponding LEDs (455-465 nm) still suffer from low efficiencies and show inferior stability. In fact, most of the efficient blue-emitting perovskite LEDs reported so far exhibit emission in the sky-blue region (470-490 nm) with a maximum EQE of up to 20%^[3]; however, such emissions are inappropriate for desired high-quality display and white lighting applications. Moreover, spectral instability in mixed-halide perovskite blue LEDs remains a significant challenge that needs to be addressed to enable the development of fully perovskite-based lighting and display technologies. In this work, we present a set of innovative material design strategies aimed at enhancing both the photoluminescence quantum yields (PLQY) and stability of pure blue-emitting (~460 nm) perovskites through compositional and ligand engineering strategies. At first, we demonstrate the design of cesium lead halide (CsPb(Cl/Br)₃) nanocrystals (NCs) via the growth-on-substrate technique using molecularly engineered phenyl-based ligands and passivating additives, and they exhibit improved PLQYs with enhanced stability over 110 days. The resulting on-substrate grown pure blue-emitting NCs exhibit PL maxima ~ 460 nm with CIE chromaticity coordinates (0.14, 0.045), closely aligning with the desired blue emitters (0.131, 0.046) as defined by Recommendation 2100. Additionally, advanced PL microscopy measurements reveal that these compositionally-engineered NCs exhibit pronounced photo-enhancement and photostability with improved optical properties, attributed to reduced crystal defects and non-radiative traps owing to the high binding affinity of the molecularly engineered ligands. Furthermore, we demonstrate the design of pure blue LEDs utilizing these pure blue-emitting perovskites composed of various molecularly engineered cations, accompanied by device engineering strategies. We also conduct the spectral stability analysis of blue LEDs, examining both emission intensity and spectral shifts under varying operational conditions (vs Voltage and Time). Based on the composition and design/processing conditions, the pure blue-emitting materials and corresponding LEDs exhibit different spectral response, behavior/performance, and stability. Overall, our work presented here demonstrates effective design strategies for pure blue emitters, provides a detailed analysis of spectral stability and shifts in compositionally engineered blue-emitting halide perovskites and their corresponding LEDs, and offers a promising pathway towards the realization of stable pure blue emitters and high-performance pure blue LEDs.