  We present a homogeneous study of blue straggler stars across ten outer halo
globular clusters, three classical dwarf spheroidal and nine ultra-faint
galaxies based on deep and wide-field photometric data taken with MegaCam on
the Canada-France-Hawaii Telescope. We find blue straggler stars to be
ubiquitous among these Milky Way satellites. Based on these data, we can test
the importance of primordial binaries or multiple systems on blue straggler
star formation in low density environments. For the outer halo globular
clusters we find an anti-correlation between the specific frequency of blue
straggler and absolute magnitude, similar to that previously observed for inner
halo clusters. When plotted against density and encounter rate, the frequency
of blue stragglers are well fitted by single trends with smooth transitions
between dwarf galaxies and globular clusters, which points to a common origin
for their blue stragglers. The fraction of blue stragglers stays constant and
high in the low encounter rate regime spanned by our dwarf galaxies, and
decreases with density and encounter rate in the range spanned by our globular
clusters. We find that young stars can mimic blue stragglers in dwarf galaxies
only if their ages are 2.5+/-0.5 Gyr and they represent ~1-7% of the total
number of stars, which we deem highly unlikely. These results point to
mass-transfer or mergers of primordial binaries or multiple systems as the
dominant blue straggler formation mechanism in low density systems.
