The fractal dimension of the flame front is an important parameter that quantifies the degree of flame folding and twisting as well as the instantaneous mass burning rate. Due to the geometric nature of flame surface folding, the fractal approach is also considered one of the most appropriate methods in power law modelling. Although biodiesel fuel is an important renewable fuel, studies on the fractal dimension of the flame during the combustion of biodiesel and its blends are limited. To address this research gap, combustion assays of biodiesel-diesel mixtures were systematically conducted within a coaxial diffusion burner and a diesel engine. High-resolution images of the evolving combustion flames were captured using a high-speed video camera. Digital image processing was applied to facilitate the extraction of key features from these images. Through the differential box counting (DBC) method, the fractal dimensions of the flame fronts were derived. A comprehensive analysis was carried out, revealing the influence of various parameters on the flame's fractal dimensions, including the biodiesel-diesel blend proportion, blend temperature, engine operational speed, and air flow rate. The results highlight that the fractal dimension undergoes a non-monotonic variation as biodiesel content increases. Specifically, the combustion of B25 blend exhibited the minimum fractal dimension, while the B100 blend showed the maximum. Further, in engine environments, peak fractal dimensions were observed to occur earlier at higher speeds (2200 r/min) with an increase in the biodiesel ratio. These insights not only bridge a critical knowledge gap in power-law modeling for biodiesel combustion but also offer a quantitative understanding of how blend ratios and engine conditions affect flame fractal properties.