Environmentally benign indium phosphide (InP) quantum dots (QDs) have emerged as promising candidates for next-generation full-color displays. Red- and green-emitting InP QDs and their quantum dot light-emitting diodes (QLEDs) have demonstrated exceptional performance, narrowing the gap with CdSe-based counterparts. In contrast, blue-emitting InP QDs and their QLED lag behind, and the challenges of their synthesis and fabrication are widely recognized in the field. Herein, based on (DMA)₃P, we propose a novel Mg-doped core/shell architecture (InP/Zn_1-xMg_xS/ZnS QDs) to achieve efficient narrow-bandwidth pure blue emission. Sustained gradient growth of Mg-doped shell layers on an initial ZnMgS monolayer achieves a dual function : (1) stepwise matching of lattice constants between core and shell, and (2) robust exciton confinement in small-size InP cores. This strategy facilitates tunable emission from 474 nm (sky blue) to 465 nm (pure blue), with a reduced full-width-at-half-maximum from 47 nm to 39 nm, alongside significantly enhanced photoluminescence quantum yield (>90%) and fluorescence lifetime (179ns). Meanwhile, to achieve much stronger storage stability, we have developed a thin ZnS shell layer in the outermost layer. This work provides a feasible and effective strategy for obtaining narrow-band pure blue-emitting InP QDs, aiming to advance environmentally friendly InP QDs and their QLEDs for full-color displays.