Colour–absolute magnitude relation for BHB stars in the Gaia EDR3 photometric system. The orange line corresponds to the relation found in this work (Eq. (1)) by fitting a polynomial on the BHB population by Xue et al. (2011) (grey points). The stars in the blue area are excluded from the fit as they are likely misidentified as BHB. Lower panels: residuals of the absolute magnitude to the fitted relation.