Impregnating organic phase change materials (PCMs) into cellulose-based aerogels is considered as an accessible and effective technology to prevent the liquid leakage issue due to the superior surface tension and capillary force. However, the poor solar-thermal conversion performance, high flammability, and low thermal conductivity still restrict the large-scale application of organic PCMs. Herein, two-dimensional (2D)-layered black phosphorus (BP) nanosheets having a superior photothermal effect were synthesized from a BP crystal through ultrasonication-assisted liquid exfoliation. Then, novel form-stable PCM composites (CBPCMs) were prepared by impregnating n-octacosane into cellulose nanofiber (CNF)/BP hybrid aerogels. The porous aerogels adequately supported the n-octacosane and prevented the liquid leakage issue. Differential scanning calorimetry (DSC) analysis demonstrated that the synthesized CBPCMs based on CNF/BP hybrid aerogels possessed extremely high n-alkane loading capacity and thermal storage density (247.0–251.6 J g⁻¹). The incorporation of BP nanosheets into the aerogels considerably increased the thermal conductivity (89.0% increase) and solar-thermal conversion and storage efficiency (up to 87.6%) of the CBPCMs. Furthermore, with the increasing content of BP nanosheets in the aerogels, the heat release rate and total heat release of the CBPCMs decreased considerably, while the LOI value and char yield increased, thus revealing the significantly improved flame retardancy of the PCM composites. In conclusion, the CBPCMs show considerable potential in solar utilisation systems.