Simple mononuclear Cu(I)-halide complexes, [CuX(PPh3)2(4-Mepy)] (X = Cl(-), Br(-), I(-); PPh3 = triphenylphosphine; 4-Mepy = 4-methylpyridine), were prepared. They exhibit blue light emission, with extremely high photoluminescence quantum yields approaching 100% in the crystals. Emission lifetime analyses and density functional theory calculations revealed that the bright blue light emission at room temperature is mainly delayed fluorescence originating from the singlet metal-to-ligand charge transfer (MLCT) state combined with the halide-to-ligand charge transfer (XLCT) state, ((1)(M + X)LCT), while that at 77 K is phosphorescence from the (3)(M + X)LCT transition state, due to the small singlet-triplet energy differences (ΔE = 940-1170 cm(-1)). The ternary ligand systems consisting of halide, bulky phosphine, and N-heteroaromatic ligands constitute inexpensive pure-blue-light-emitting materials, which can be fabricated by facile procedures such as simple manual grinding.