  In this work, we study the luminosity that results from the conversion of QCD
axion particles into photons in the magnetic field of the plasma accreting onto
black holes (BHs). For the luminosities to be large two conditions need to be
met: i) there are large numbers of axions in the PBH surroundings as a result
of the so-called superradiant instability; ii) there exists a point inside the
accreting region where the plasma and axion masses are similar and there is
resonant axion-photon conversion. For BHs accreting from the interstellar
medium in our galaxy, the above conditions require the black hole to have
subsolar masses and we are therefore led to consider a population of primordial
black holes (PBHs). In the conservative window, where we stay within the
non-relativistic behavior of the plasma and neglect the possibility of
non-linear enhancement via magnetic stimulation, the typical frequencies of the
emitted photons lie on the low-radio band. We thus study the prospects for
detection using the LOFAR telescope, assuming the PBH abundance to be close to
the maximal allowed by observations. We find that for PBH and QCD axion with
masses in the range $10^{-5}-10^{-4}\, M_\odot$ and $4 \times 10^{-8}$ and $4
\times 10^{-7}$ eV, respectively, the flux density emitted by the closest PBH,
assuming it accretes from the warm ionized medium, can be detected at the LOFAR
telescope. Coincidently, the PBH mass range coincides with the range that would
explain the microlensing events found in OGLE. This might further motivate a
dedicated search of these signals in the LOFAR data and other radio telescopes.
