Does anyone else leave black stains everywhere

I've searched but only seen posts about an Evo doing this so does anyone know what is causing this?

 

I removed the converter from a stock exhaust manifold I had on hand and installed it, only cause I kept getting a P0421 code, "but I installed a whole new exhaust system on my car" so I'm only running a high flow cat underneath, well soot is not sticking on my bumper anymore and the long soot stain was cut to almost nothing so hell yeah, oh P0421 hasn't been back on yet five months so far