Indeed, that has to do with the heat of the exhaust gasses. Since this year, the cars must have their exhaust on top of the side pods. Most teams (like red bull) try to let the hot airflow ‘stick’ to the side pod, to guide it to the diffuser. Because these gasses are extremely hot (say 800 degrees), the side pods need to be protected with some kind of heat resistor, which is the black part you see at the back of the side pods.
