  Newborn black holes in collapsing massive stars can be accompanied by a
fallback disk. The accretion rate is typically super-Eddington and strong disk
outflows are expected. Such outflows could be directly observed in some failed
explosions of compact (blue supergiants or Wolf-Rayet stars) progenitors, and
may be more common than long-duration gamma-ray bursts. Using an analytical
model, we show that the fallback disk outflows produce blue UV-optical
transients with a peak bolometric luminosity of ~10^(42-43) erg s^-1 (peak
R-band absolute AB magnitudes of -16 to -18) and an emission duration of ~ a
few to ~ 10 days. The spectra are likely dominated intermediate mass elements,
but will lack much radioactive nuclei and iron-group elements. The above
properties are broadly consistent with some of the rapid blue transients
detected by Pan-STARRS and PTF. This scenario can be distinguished from
alternative models using radio observations within a few years after the
optical peak.
