  We consider the formation of horizon-size primordial black holes (PBH's) from
pre-existing density fluctuations during cosmic phase transitions. It is
pointed out that the formation of PBH's should be particularly efficient during
the QCD epoch due to a substantial reduction of pressure forces during
adiabatic collapse, or equivalently, a significant decrease in the effective
speed of sound during the color-confinement transition. Our considerations
imply that for generic initial density perturbation spectra PBH mass functions
are expected to exhibit a pronounced peak on the QCD-horizon mass scale $\sim 1
M_{\odot}$. This mass scale is roughly coincident with the estimated masses for
compact objects recently observed in our galactic halo by the MACHO
collaboration. Black holes formed during the QCD epoch may offer an attractive
explanation for the origin of halo dark matter evading possibly problematic
nucleosynthesis and luminosity bounds on baryonic halo dark matter.
