  Supermassive black holes in active galactic nuclei launch relativistic jets,
as indicated by observed superluminal radio blobs. The energy source of these
jets is widely discussed in the theoretical framework of Blandford-Znajek
process, the electromagnetic energy extraction from rotating black holes (BHs),
while formation mechanism of the radio blobs in the
electromagnetically-dominated jets has been a long-standing problem. Recent
high-resolution magnetohydrodynamic simulations of magnetically arrested disks
exhibited magnetic reconnection in a transient magnetically-dominated part of
the equatorial disk near the BH horizon, which led to a promising scenario of
efficient MeV gamma-ray production and subsequent electron-positron pair
loading into BH magnetosphere. We develop this scenario to build a theoretical
framework on energetics, timescales and particle number density of the
superluminal radio blobs and discuss observable signatures in other wavebands.
We analytically show that the non-thermal electrons emit broadband photons from
optical to multi-MeV bands. The electron-positron pairs produced in the
magnetosphere are optically thick for synchrotron-self absorption, so that the
injected energy is stored in the plasma. The stored energy is enough to power
the superluminal radio blobs observed in M87. This scenario predicts rather dim
radio blobs around Sgr A*, which are consistent with no clear detection by
current facilities. In addition, this scenario inevitably produces strong X-ray
flares in a short timescale, which will be detectable by future X-ray
satellites.
