Black Smoke from 01 GT

Last weekend me and my brother were driving around in our 01 stangs when a S10 got behind us and wanted to have some fun. I played around with him first then my brother got next to him and from about 35mph they both floored it. Of course the S10 was no match , but anyhow I noticed when my brother was at WOT there was a decent amount of black smoke coming out of his pipes. It doesn't come out under normal driving just when he lays on the gas. I've never noticed this with mine. His mods are LT headers, catted x-pipe, 1 chamber flows, mac straight shot, 80mm MAF, accufab TB and plenum, 4.10s, and a diablosport predator. Is this black smoke normal at WOT and if not what could cause it?