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Long term stability of the black perovskite phase of CsPbI3 nanocrystals under ambient conditions is an important challenge for their optoelectronic applications in real life. The nanocrystalline size is found to improve the stability of the black phase at room temperature. Furthermore, doping Mn is proposed to improve the stability of the black perovskite phase of CsPbI3 nanocrystals (NCs). However, the undoped and Mn-doped CsPbI3 NCs are prepared in different batches under somewhat different synthesis conditions thus obliterating the role of Mn in the stability of the black phase of CsPbI3 NCs. Here, we elucidate the effect of Mn doping on the surface and lattice energy of CsPbI3 NCs, stabilizing the black phase. For this purpose, we employ a postsynthesis doping strategy which has an advantage that the initial host remains the same for both undoped and Mn-doped samples. Uncertainties in the size/shape, surface energy, and structure through direct synthesis of undoped and Mn-doped NCs in different batches can be neglected in our postsynthesis doping strategy, allowing us to study the effect of dopants in a more controlled manner. Our postsynthesis Mn-doping in CsPbI3 NCs shows that the black phase stability under ambient conditions improves from few days for the undoped sample to nearly a month's time for the Mn-doped sample. We found that though surface passivation with a dopant precursor improves both colloidal and phase stability of black CsPbI3 NCs, it is the contraction of the lattice upon Mn-doping that mainly stabilizes the films of black phase CsPbI3 NCs. Similarly, we found that Mn-doped CsPbBr3 NCs show improved ambient stability of photoluminescence compared to the undoped sample.