  Abridged... Blue stragglers (BSS) are thought to be the product of either
primordial or collisional binary systems. In the context of dwarf spheroidal
galaxies it is hard to firmly disentangle a genuine BSS population from young
main sequence (MS) stars tracing a ~1-2 Gyr old star forming episode. Assuming
that their blue plume populations are made of BSS, we estimate the BSS
frequency for 8 Local Group non star-forming dwarf galaxies, using a
compilation of ground and space based photometry.
  Our results can be summarized as follows: (i) The BSS frequency in dwarf
galaxies, at any given Mv, is always higher than that in globular clusters of
similar luminosities; (ii) the BSS frequency for the lowest luminosity dwarf
galaxies is in excellent agreement with that observed in the Milky Way halo;
and most interestingly (iii) derive a statistically significant
anti-correlation between the BSS frequency and the galaxy Mv.
  The low density, almost collision-less, environments of our dwarf galaxy
sample allow us to infer (i) their very low dynamical evolution; (ii) a
negligible production of collisional BSS; and consequently (iii) that their
blue plumes are mainly made of primordial binaries. The dwarf galaxies
anti-correlation can be used as a discriminator: galaxies obeying the
anti-correlation are more likely to possess genuine primordial BSS rather than
young main sequence stars.
