This work reports on low-temperature inorganic CsPbI₃ perovskite nanostructures synthesized as the active black phase, without the additional use of organic ligands and based only on CsI and PbI₂ precursors. This new method is based on the “inverse temperature crystallization” (ITC) phenomenon where dissolved lead salts tend to form nucleation grains at high temperatures. This methodology allows the conversion temperature of the CsPbI₃ black phase to be reduced without the use of additives or anti-solvent treatment. We use small angle X-ray scattering (SAXS), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and photoluminescence (PL) measurements to characterize the precursor solutions at different heating times to understand the nature of the observed CsPbI₃ nanoparticles (NPs). Heating the solution for 192 hours shows the high-quality black active phase of CsPbI₃ NPs after evaporation of the solvent in the solid state. This allows us to form a film of CsPbI₃ in its photoactive phase at a low temperature (T = 55 °C) within a few minutes using no additives or antisolvent treatment. We use the dispersion of CsPbI₃ nanostructures to fabricate black-phase CsPbI₃ perovskite-based solar cells on a mesoporous TiO₂ structure showing a power conversion efficiency of 7.3%.