Abstract: Polymer-based thermally conductive films have attracted extensive attention in thermal management and heat dissipation of electronic devices due to their simple processing and high design flexibility. Graphene nanoplatelets (GNPs) possess outstanding in-plane thermal conductivity and can remarkably enhance the in-plane heat transport of polymer matrices; however, their contribution to through-plane thermal conductivity is rather limited. Existing strategies that simultaneously improve both the in-plane and through-plane thermal conductivities of graphene-based composite films often suffer from complicated processing and high cost, which restrict their large-scale application. In this work, carbon black (CB) was introduced into a GNP/PVA system to fabricate CB-GNP/PVA composite films. Owing to its nanoscale particle size and isotropic thermal transport, CB can form a “point-to-plane” complementarity with the two-dimensional GNPs, thereby synergistically constructing an efficient through-plane heat-conduction network. The results show that the maximum through-plane thermal conductivity of the composite films reaches 2.11 W·m⁻¹·K⁻¹, representing a 486.1% increase compared with the GNP/PVA film without CB. Meanwhile, the composite films maintain excellent in-plane thermal conductivity, with a maximum value of 25.7 W·m⁻¹·K⁻¹. In addition, the incorporation of CB significantly improves the tensile and electrical properties of the films, achieving a maximum tensile strength of 51.04 MPa and an electrical conductivity of 1756.2 S/m. Benefiting from the enhanced in-plane and through-plane thermal conductivities, the composite films were successfully applied to thermal management of a heating plate and a chip-on-board (COB) module, demonstrating pronounced heat-dissipation enhancement. This study provides a feasible approach for developing high-performance flexible thermal management materials.