Chocolate is definitely not great for dogs, but it's the bakers chocolate that's the most toxic.
Vomiting and refusing to eat is scary and warrants a trip to the vet. Dogs can very quickly become dehydrated.
My oldest dog got into a bag of Dove chocolates years ago that my boyfriend carelessly left lying on the kitchen counter. We went out for the evening, and came back and the bag was on the floor empty. I accused him of eating all the chocolate.
Miss Ivory used to be quite the counter surfer. The next day we figured out that she actually snarfed down the chocolate (foil wrappers and all). The next day on our walk, she had silvery shiny poo. Everything passed without incident. It's one of the many times in her almost 16 years that Miss Ivory dodged a bullet.

That incident didn't cause her any ill effects--no vomiting, no diarrhea...nothing. She got a vet visit anyway complete with bloodwork, and nothing was abnormal. Your mileage may vary.
