Control of material defects is an effective tool to improve the light-conversion efficiency of thin-film solar cell materials. Here, using first-principles calculations, we report a significant enhancement of optical and transport properties upon introducing a single vacancy into blue phosphorene quantum dots. We employ the generalized gradient approximation with the Perdew-Burke-Ernzerhof and the hybrid density functional theory model Becke, 3-parameter, and Lee-Yang-Parr (B3LYP) functionals as exchange–correlation functionals, to compute the equilibrium structure, vibration spectra and optoelectronic properties. We also evaluate the impact of a single vacancy on device performance by placing the blue phosphorene dots between two gold electrodes to mimic molecular junctions using the non-equilibrium Green function formalism with density functional based tightbinding methods. We find that we can effectively tune the electronic band gaps of these quantum dots from 4.03 eV to 3.73 eV (B3LYP values at the 6–311++G(d,p) levels) by cutting the blue phosphorene sheet to various quantum dot shapes. Furthermore, in the presence of a single vacancy, the band gaps shrink significantly to between 1.91 eV and 1.78 eV (B3LYP values at the 6–311++G(d,p) levels), due to the formation of polaronic states induced by the vacancy, resulting in a dramatic down shift of the conduction band towards the valence band. These polaronic states, on the one hand, induce more new absorption frequencies in the visible light range; on the other hand, they reduce or increase the current passing through the quantum dot molecular junctions depending on its morphology. Our results highlight the sensitivity to defects of blue phosphorene quantum dots in applications for solar cell devices.