While humans metabolize the theobromine in chocolate quickly, dogs have difficulty doing so and it can cause a buildup of toxicity in their systems. Size matters, of course, and a larger dog can handle more chocolate than a small dog without dangerous consequences, and there is a great online calculator at petmd.com you can use to determine if you need to take your dog to the emergency vet. Warning signs of chocolate poisoning include GI upset, vomiting, restlessness or hyperactivity, increased heart rate, diarrhea, rapid breathing, muscle tension, and seizures. Dark chocolate contains more theobromine than other types of chocolate, and symptoms can begin to manifest within 2-24 hours after ingestion.
Best thing to do is to keep those lovely chocolates in a closed cupboard, protecting your dog from risk. Easier to never have the problem at all than to have to try and deal with it and the expensive bills that can go along with it.
I may sell pet food, but I also have a Master's Degree in History. Anyone want to talk British Navy stuff? No? You just want the pet food stuff? Ok...