Lead halide perovskites exhibit a very wide color gamut due to their extremely narrow emission spectra, typically characterized by a full-width at half-maximum (FWHM) of less than 20 nm. Significant advancements have been made in developing highly efficient and stable green, red, and near-infrared perovskite light-emitting diodes (PeLEDs). However, achieving efficient and stable pure blue-emitting PeLEDs remains a significant challenge. In this work, we successfully synthesized monoanionic octyl-phosphonate capped CsPbBr3 nanoplatelets (OPA-NPLs) using a combination of octyl-phosphonic acid and oleylamine at room temperature, diverging from common approaches that necessitate complex high-temperature methods, such as hot injection, to accommodate short-chain ligands. The OPA-NPLs exhibit pure blue photoluminescence at 462 nm with a FWHM of 14 nm. Compared with CsPbBr3 nanoplatelets synthesized using oleic acid, OPA-NPLs demonstrate significantly improved thermal stability and higher photoluminescence quantum yield (PLQY) of 90%. Additionally, we introduced Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), a conjugated polyelectrolyte material, as a hole transport layer. This facilitated energy transfer between PFN-Br and the CsPbBr3 nanoplatelets. The resulting device demonstrated an electroluminescence peak at 462 nm, an extremely narrow FWHM of 14 nm, and a maximum external quantum efficiency (EQE) of 4%. Notably, the device maintained pure blue emission without spectral peak shift even during degradation caused by excess joule heating.