Distribution of F_γ in the 100 MeV−100 GeV band for all 351 BZBs in our selected sample. As in Fig. 2, the blue histogram indicates the X-ray merged event file with at least one X-ray source detected with S/N >  3, and in red we show those without an X-ray detection. The solid black line corresponds to the threshold above which 100% of the selected BZBs has an X-ray counterpart, while the dashed and dot-dashed black lines mark the 98% and 96% thresholds, respectively.