  The recent inference of a 70M_sun black hole (BH) in the Galactic, detached
binary LB-1 has sparked cross-disciplinary debate since a stellar remnant of
such large mass is well above what can be expected from stellar-evolutionary
theory, especially in an enriched environment like that of the Milky Way. This
study focusses on the possibilities of formation of extraordinarily massive BHs
at solar and globular cluster (GC)-like metallicities via evolution of massive
stellar binaries. A population-synthesis approach is followed utilizing the
recently updated BSE program. BHs in the mass range of 50M_sun-80M_sun could be
formed at the solar metallicity only if a large fraction, >70%, of matter is
allowed to accrete onto a low-mass BH, in a BH-star merger product (a "black
hole Thorne-Zytkow object"; BH-TZO). Their counterparts at GC-like
metallicities can reach 100 M_sun. Although post-accretion BHs can, generally,
be expected to be of high spin parameter, they can potentially be of low spin
in the case of a BH-TZO. This spin aspect remains speculative in this work and
deserves detailed hydrodynamic studies.
