  We present theoretical models of black hole (BH) populations in young stellar
environments, such as starbursts and young star clusters. Using a population
synthesis approach we compute the formation rates and characteristic properties
of single and binary BHs for various representative ages and choices of
parameters. We find that most of the BHs (typically 80% for an initial 50%
binary fraction) are single, but with many originating from primordial binaries
(which either merged into a single massive star or were disrupted following a
supernova explosion). A smaller but significant fraction (typically 20%) of the
BHs remain in binary systems. Main-sequence stars are the most frequent BH
companions, but massive BH--BH binaries are the next most numerous group. The
most massive BHs found in our simulations reach 80 Msun, and are formed through
mergers of massive binary components. If formed in a dense star cluster such a
massive stellar BH may become the seed for growth to a more massive
(``intermediate-mass'') BH. Although we do not include dynamical interactions,
our results provide realistic initial conditions for N-body simulations of
dense star clusters (e.g., globular clusters) including primordial BHs.
