The synthesis of perovskite-based blue light-emitting particles is valuable for several applications as the excellent optical properties and performances of the constituting materials associated with multi-exciton generation can be exploited. However, the preparation of perovskite precursors requires high temperatures, resulting in a complex manufacturing process. This paper proposes a one-pot method to synthesize CsPbClBr2 blue light-emitting quantum dots (QDs). In the case of nonstoichiometric precursor synthesis, the CsPbClBr2 QDs coexisted with additional products. The solvent for synthesizing mixed perovskite nanoparticles (containing chloride) was selected by mixing dimethylformamide (DMF) and/or dimethyl sulfoxide (DMSO) in different ratios. When only DMF was used with the stoichiometric CsBr and PbX2 (X = Cl, Br) ratio, the quantum yield was 70.55%, and superior optical properties were achieved. Moreover, no discoloration was observed for 400 h, and a high photoluminescence intensity was maintained. When deionized water was added to form a double layer with hexane, the luminescence was maintained for 15 days. In other words, the perovskite did not easily decompose even when in contact with water, which suppressed the release of Pb2+, which are heavy metal atoms in the structure. Overall, the proposed one-pot method for all-inorganic-based perovskite QDs provides a platform for synthesizing superior blue light-emitting materials.