Photothermal conversion can promote plastic depolymerization (chemical recycling) through light-to-heat conversion. The highly localized temperature gradient on photothermal agent surface allows selective heating with spatial controls not observed with bulk heating. However, identifying practical photothermal agents that are easily incorporated and reusable can be challenging. Interestingly, the rarely recycled black plastics containing carbon black is a potential candidate for photothermal conversion recycling. Herein, we use photothermal conversion to depolymerize commercial polystyrene plastics back into styrene monomers using the pigment in black plastics. Synthesized polystyrene-carbon black composites were depolymerized under white LED light irradiation, producing styrene monomer in up to 60 % yield. We demonstrate the recyclability of monomer and carbon black for a fully circular plastics economy. Ultimately, commercial black polystyrene samples are successfully converted to styrene through photothermal depolymerization without additional additives, with yields up to 70 % under focused solar irradiation in just 5 minutes. Broadly, this sustainable method holds the potential to actualize a closed-loop economy of black plastics.