Boron nitride nanosheets (BNNS), also known as "white graphene", combine a two-dimensional morphology with a high and anisotropic refractive index (RI), enabling strong and direction-dependent light-nanofiller interactions. While offering multifunctional reinforcement, BNNS also alter photon transport during photopolymerisation of dental resin-based composites (RBCs). Here, experimental RBCs containing increasing BNNS loadings (0 wt.%–10.0 wt.%) together with RI-matched glass fillers were formulated to elucidate how BNNS-induced optical anisotropy and RI mismatch govern light propagation, curing efficiency, and macroscopic optical properties. BNNS were characterised by scanning electron microscopy (SEM), atomic force microscopy (AFM) and dynamic light scattering (DLS). The RI of fillers and resin were measured optically. The light curability and optical performance of BNNS-reinforced RBCs, including light transmittance, degree of conversion (DC), depth of cure (DoC), translucency, flexural strength (FS), flexural modulus (FM), and fracture microstructure were evaluated. BNNS exhibited a platelet-like morphology with a high aspect ratio and a substantially higher RI than the surrounding RBCs components, generating orientation-dependent scattering and refraction. Increasing BNNS loading with deteriorated dispersion and increased agglomeration altered the light-scattering regime, reduced photon flux at depth, resulting in decreased transmittance, DoC, and translucency while increasing FM. At low loadings, partial scattering enhanced local light distribution, producing peak DC and FS at 0.5 wt.%, whereas higher BNNS caused severe optical attenuation, drastically reduced DC and slightly decreased FS at 10.0 wt.%. These findings identify potential ranges for BNNS loadings, tailored to applications for repair of enamel, dentin replacement or masking. These established constraints for formulation of light-curable tooth-coloured nanocomposites.