The development of environmentally friendly and high-performance biolubricants is crucial for sustainable material processing applications. In this study, castor oil-based nanolubricants were formulated by adding boron nitride nanoparticles (up to 1.0 wt%) and black cumin (Nigella sativa) oil (up to 60 mg/mL) as a natural antioxidant additive. Oleic acid was used for surface modification of boron nitride nanoparticles to improve dispersion stability. Comprehensive characterizations, including FT-IR, UV–VIS, viscosity, hot oil oxidation test (HOOT), thermal conductivity, and thermogravimetric analysis (TGA), were performed to evaluate structural, oxidative, and thermal behaviors. The results showed that the co-addition of boron nitride nanoparticles and black cumin oil significantly improved oxidation stability up to the optimum concentration, and pro-oxidant effects were observed beyond this concentration. 0.05 wt% boron nitride containing 30 mg/mL black cumin oil demonstrated superior oxidative stability by minimizing viscosity changes and delaying the oxidation temperature. A multi-criteria decision-making model revealed that a formulation containing 0.05 wt% boron nitride and 10 mg/mL black cumin oil was the optimum solution for improving dispersion stability, viscosity, oxidation stability, thermal conductivity, and oxidation onset temperature. The findings indicate that the synergistic effect of the natural antioxidant of black cumin oil and boron nitride nanoparticles in improving the oxidation stability of castor oil-based lubricants offers a sustainable and high-value alternative.