• Presence of ZnO NP decreases field-induced birefringence. • Transition temperature decreases but the BP exists for a narrow temperature range compared to pure. • The Kerr constant (K) decreases and is temperature-dependent. • K varies with temperature and is essentially doubled for the system with the nanoparticles. We report the influence of a small concentration (0.1 wt%) of ZnO nanoparticles (NPs) on the birefringence of the liquid crystal Blue phase. The measurements carried out using the field-induced birefringence technique show that the inclusion of NP diminishes the magnitude of the Kerr constant. We propose that the field-induced disclination networks act as trapping centres for the NPs and enhance the elastic constant leading to a diminution in the birefringence.