Mass–metallicity relation of the samples of Cardamone et al. (2009) GPs (green rhombuses), Izotov et al. (2011) BDGs (red X’s), Yang et al. (2017) BBs (blue squares), and the spectroscopic subsample of this work’s BBs (purple circles). Uncertainties are not shown for the first two samples for clarity. Colored curves show the mass-SFR-metallicity relation from Eq. 2 of Curti et al. (2020) for a range of log (SFR/M_⊙ yr⁻¹) = 1.2 (green) to log (SFR/M_⊙ yr⁻¹) = − 0.75 (blue).