The black areas on the fenders are actually recesses in the body mesh.
This car was originally a GTR2 conversion and the various "Aero" packages that could be changed via the Genstring were omitted (all but the pro variation) in this rfactor version, which left the air intakes open in the fenders.
I've taken just the fender fillers from one of the GTR2 Aero meshes and added them to the Aero variation used on this rfactor model. Just extract the GMT file into the main Vehicles/GT2/Ferrari_458GTC folder. Have fun.