  We consider implications of high-energy neutrino emission from blazar flares,
including the recent event IceCube-170922A and the 2014-2015 neutrino flare
that could originate from TXS 0506+056. First, we discuss their contribution to
the diffuse neutrino intensity taking into account various observational
constraints. Blazars are likely to be subdominant in the diffuse neutrino
intensity at sub-PeV energies, and we show that blazar flares like those of TXS
0506+056 could make <1-10 percent of the total neutrino intensity. We also
argue that the neutrino output of blazars can be dominated by the flares in the
standard leptonic scenario for their gamma-ray emission, and energetic flares
may still be detected with a rate of <1 per year. Second, we consider
multi-messenger constraints on the source modeling. We show that luminous
neutrino flares should be accompanied by luminous broadband cascade emission,
emerging also in X-rays and gamma-rays. This implies that not only gamma-ray
telescopes like Fermi but also X-ray sky monitors such as Swift and MAXI are
critical to test the canonical picture based on the single-zone modeling. We
also suggest a two-zone model that can naturally satisfy the X-ray constraints
while explaining the flaring neutrinos via either photomeson or hadronuclear
processes.
