  A population of black holes (BHs) at high redshifts (z>6) that contributes
significantly to the ionization of the intergalactic medium (IGM) would be
accompanied by the copious production of hard (>10keV) X-ray photons. The
resulting hard X-ray background would redshift and be observed as a present-day
soft X-ray background (SXB). Under the hypothesis that BHs are the main
producers of reionizing photons in the high-redshift universe, we calculate
their contribution to the present-day SXB. Our results, when compared to the
unresolved component of the SXB in the range 0.5-2 keV, suggest that accreting
BHs (be it luminous quasars or their lower-mass ``miniquasar'' counterparts)
did not dominate reionization. Distant miniquasars that produce enough X-rays
to only partially ionize the IGM to a level of at most 50% are still allowed,
but could be severely constrained by improved determinations of the unresolved
component of the SXB.
