Formation of BBH mergers is a probe of internal chemical mixing in stars and of the stellar structure. We plot the minimum BBH delay times from SMT evolution given the donor mass (at ZAMS) and radius (at RLOF) for two variations in the efficiency of semiconvective mixing in stars (α_semiconv.). Only cases with the BBH delay time below ≲13.7 Gyr (yellow-brown regions) are potential GW sources. The range of donors that may lead to BBH mergers is severely reduced in the variation with low efficiency of semiconvection (right panel) and it shows a preference for high-mass donors (≳40 M_⊙). This is because the chemical structure of donor stars determines the conditions for delayed-dynamical instability and therefore affects the critical mass ratios, the minimum post-SMT separations, and BBH delay times (Sect. 4.2). This is different from the assumptions on the orbital AM loss that only affects the q_crit (Fig. 5). The difference between stellar models in both panels is closely connected to the blue supergiant problem (Sect. 4.3). The thick blue line shows the transition from MS to post-MS donors. Several main types of donors are labeled (Hertzsprung gap, core-He burning, convective envelopes, He-rich envelopers); see text. Based on MESA models of BH+O-star binaries evolving through SMT at the critical mass ratio q = q_crit. See Fig. D.2 for post-SMT separations.