I was just at Thuder Hill Raceway's open track session today and noticed the same thing on my tires. Under extreme heat situations, it seems as though the tire will melt, causing one of two things - the first being the patches you're talking about and the second being tire chunks flying up behind your car. You probably have black marks on the body behild the rear wheels as a result of the latter. Not to worry, they rub off fairly easily. Since I was running on the track with other cars, I actually had these black rubber marks all over the front of my car as well. I guess thats the price we pay for the fun that we have :)