If you're just talking about a 1-time thing, or a special treat, no biggie. If the amount you're giving your dog is less than 2-4% of its body weight, you shouldn't have any issue at all. If you're talking about overall diet balance, and feeding a particular kind of meat every single day, feeding only dark turkey meat isn't something you'll want to do.
I don't know of anything inherently bad about dark meat turkey that would harm a dog. Dark meat of any animal typically has a higher fat content so you want to be careful how much you're feeding and how it fits into the overall diet. I've bought ground dark turkey meat and added a 1/3 cup to my dogs' dinners once a week with no problem.