Wipe it with Armour All Original.
Use small clothes, soak it with armour all liquids, and start wiping on both sides. Continue doing so until the black residue become less apparent.
I do this with my nitro cars, and my nitro belts last 2 years. ( unless I got into stone chipping situation ).
On Road : Xray NT1 , RX8 , T4 || Mugen Seiki MRX4-R , MTX4 , MTX3 prospec || Tamiya M03 , M04 || Top Racing Sabre FD2
Off Road : Xray XT8 || Hobao Hyper 8
Radio : Futaba 3PKS || KO propo EX-10 eurus