Not sure where you get the black from, probly the rubber as seen as its sat on the ash stuff it gets covered in it so when it's harrowed it moves about and ends up on the top of the rubber eventually and then gets wet and if your unlucky enough to fall off gets on you!
No idea what the rubbers from it's all sorts of random shaps like little circle bits and flat bits, bits that look like there from tyres, is very comfy though if you do fall off!!