Distributions of observed blue supergiants in the M_bol versus plane (triangles, see references in Fig. 2). The shaded zones indicate where the blue supergiants are predicted according to the stellar models. Left panel: the observations correspond to stars with a metallicity equal to or larger than about half solar, and the shaded areas show the predictions for the Z = 0.014 stellar models (rotating and non-rotating with standard mass-loss rates). Right panel: the observations correspond to stars with a metallicity lower than about half solar, and the shaded areas show the predictions for the rotating and non-rotating Z = 0.002 stellar models with standard mass-loss rates. The blue dots show the very restricted zone where Group-2 BSGs lie in this diagram, see lower panels of Fig. 5.