Do you mean the meat itself is turning black? This is not unusual for brisket or pork shoulder, it often looks very dark or black, almost burned looking, especially if you use a rub and/or baste that contains sugar.
Or are you saying that a black substance is depositing on the surface of the meat? Can you wipe it off with your finger or a paper towel? Maybe that's creosote? I've never had a black substance deposit on my meat, but I have found a grey, soot-like substance on chicken a couple of times. Is that creosote?
Thoughts anyone?
Regards,
Chris