  We have studied the candidate optical counterparts and the stellar population
in the star-forming complex around a bright ULX in NGC4559, using HST/WFPC2,
XMM-Newton/OM, and ground-based data. We find that the ULX is located near a
small group of OB stars. The brightest point source in the Chandra error circle
is consistent with a single blue supergiant of mass ~ 20 M_sun and age ~ 10
Myr. A few other stars are resolved inside the error circle: mostly blue and
red supergiants with masses ~ 10-15 M_sun and ages ~ 20 Myr. This is consistent
with the interpretation of this ULX as a black hole (BH) accreting from a
high-mass donor star in its supergiant phase, via Roche-lobe overflow. The
observed optical colors and the blue-to-red supergiant ratio suggest a low
metal abundance: 0.2 <~ Z/Z_sun <~ 0.4 (Padua tracks), or 0.05 <~ Z/Z_sun <~
0.2 (Geneva tracks). The age of the star-forming complex is <~ 30 Myr. H-alpha
images show that this region has a ring-like appearance. We propose that it is
an expanding wave of star formation, triggered by an initial density
perturbation, in a region where the gas was only marginally stable to
gravitational collapse. A possible trigger was the collision with a satellite
dwarf galaxy, visible a few arcsec north-west of the complex, going through the
gas-rich outer disk of NGC4559. The X-ray data favour a BH more massive (M > 50
M_sun) than typical Milky Way BH candidates. The optical data favour a
``young'' BH originating in the recent episode of massive star formation;
however, they also rule out an association with young massive star clusters. We
speculate that other mechanisms may lead to the formation of relatively massive
BHs (~ 50-100 M_sun) from stellar evolution processes in low-metallicity
environments, or when star formation is triggered by galactic collisions.
