  Detonation initiation in a reactive medium can be achieved by an externally
created shock wave. Supersonic flow onto a gravitating center, known as
Bondi-Hoyle-Lyttleton (BHL) accretion, is a natural shock wave creating
process, but, to our knowledge, a reactive medium has never been considered in
the literature. Here, we conduct an order of magnitude analysis to investigate
under which conditions the shock-induced reaction zone recouples to the shock
front. We derive three semianalytical criteria for self-sustained detonation
ignition. We apply these criteria to the special situation where a primordial
black hole (PBH) of asteroid mass traverses a carbon-oxygen white dwarf (WD).
Since detonations in carbon-oxygen WDs are supposed to produce normal
thermonuclear supernovae (SNe Ia), the observed SN Ia rate constrains the
fraction of dark matter (DM) in the form of PBHs as $\log_{10}(f_{\rm PBH})<
0.8 \log_{10}(M_{\rm BH}/3\times 10^{22}{\rm g})$ in the range
$10^{21}\!-\!10^{22}$g ($10^{20}\!-\!10^{22}$g) from a conservative
(optimistic) analysis. Most importantly, these encounters can account for both
the rate and the median explosion mass of normal sub-Chandrasekhar SNe Ia if a
significant fraction of DM is in the form of PBHs with mass $10^{23}$g.
