  The merger of binary neutron stars (NS-NS) as the progenitor of short
Gamma-ray bursts (GRBs) has been confirmed by the discovery of the association
of the gravitational wave (GW) event GW170817 with GRB 170817A. However, the
merger product of binary NS remains an open question. An X-ray plateau followed
by a steep decay ("internal plateau") has been found in some short GRBs,
implying that a supra-massive magnetar operates as the merger remnant and then
collapses into a newborn black hole (BH) at the end of the plateau. X-ray bump
or second-plateau following the "internal plateau" is considered as the
expected signature from the fallback accretion onto this newborn BH through
Blandford-Znajek mechanism (BZ). At the same time, a nearly isotropic wind
driven by Blandford-Paynemechanism (BP) from the newborn BH's disk can produce
a bright kilonova. Therefore, the bright kilonova observation for a short GRB
with "internal plateau" (and followed by X-ray bump or second-plateau) provides
further evidence for this scenario. In this paper, we find that GRB 160821B is
a candidate of such a case, and the kilonova emission of GRB 160821B is
possibly powered by the BP wind from a newborn BH. Future GW detection of GRB
160821B-like events may provide further support to this scenario, enable us to
investigate the properties of the magnetar and the newborn BH, and constrain
the equation of state of neutron stars.
