What powers the hard, non-thermal X-rays from accreting black holes is an unsolved mystery. We address the underlying question of what energizes the electrons of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses. We perform first-principle particle-in-cell simulations of 'radiative' magnetic reconnection—subject to IC cooling—in magnetically dominated (σ>>1) electron-positron plasmas, and for the first time, in mildly-magnetized (σ~1) electron-ion plasmas. We find that the electron energy spectrum is dominated by a ~Maxwellian-shaped peak at trans-relativistic energies (~100 keV). This results primarily from the bulk motions of plasmoids composed of electrons that are cooled down to non-relativistic energies. Thich makes the oft-invoked paradigm of 'thermal Comptonization' by hot electrons unfeasible. In electron-ion corona, the radiatively-cooled electrons are not re-heated by the hot ions via either collisional or collisionless modes of energy transfer. On the other hand, Comptonization by the bulk motions of the plasmoid chain can naturally explain the non-thermal spectra of accreting compact objects. Our latest global resistive GRMHD simulations also reveal the formation of trans-relativistic plasmoid chains that lend support to our kinetic model. We complement our particle-in-cell simulations with Monte-Carlo calculations of the transfer of seed soft photons through the reconnection layer, and produce synthetic X-ray spectral models.