  We have run 600 N-body simulations of intermediate-mass (~3500 Msun) young
star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1
Zsun). The simulations include the dependence of stellar properties and stellar
winds on metallicity. Massive stellar black holes (MSBHs) with mass >25 Msun
are allowed to form through direct collapse of very massive metal-poor stars
(Z<0.3 Zsun). We focus on the demographics of black hole (BH) binaries that
undergo mass transfer via Roche lobe overflow (RLO). We find that 44 per cent
of all binaries that undergo an RLO phase (RLO binaries) formed through
dynamical exchange. RLO binaries that formed via exchange (RLO-EBs) are powered
by more massive BHs than RLO primordial binaries (RLO-PBs). Furthermore, the
RLO-EBs tend to start the RLO phase later than the RLO-PBs. In metal-poor SCs
(0.01-0.1 Zsun), >20 per cent of all RLO binaries are powered by MSBHs. The
vast majority of RLO binaries powered by MSBHs are RLO-EBs. We have produced
optical color-magnitude diagrams of the simulated RLO binaries, accounting for
the emission of both the donor star and the irradiated accretion disk. We find
that RLO-PBs are generally associated with bluer counterparts than RLO-EBs. We
compare the simulated counterparts with the observed counterparts of nine
ultraluminous X-ray sources. We discuss the possibility that IC 342 X-1, Ho IX
X-1, NGC 1313 X-2 and NGC 5204 X-1 are powered by a MSBH.
