  We propose a mechanism that can convert a sizeable fraction of neutron stars
into black holes with mass $\sim 1M_\odot$, too light to be produced via
standard stellar evolution. We show that asymmetric fermionic dark matter of
mass $\sim$ TeV, with attractive self-interaction within the range that
alleviates the problems of collisionless cold dark matter, can accumulate in a
neutron star and collapse, forming a seed black hole that converts the rest of
the star to a solar mass black hole. We estimate the fraction of neutron stars
that can become black holes without contradicting existing neutron star
observations. Like neutron stars, such solar mass black holes could be in
binary systems, which may be searched for by existing and forthcoming
gravitational wave detectors. The (non-)observation of binary mergers of solar
mass black holes may thus test the specific nature of the dark matter.
