I am so sorry Jerry! I meant "Jerry", not "Margot"! (mixed up ! lol)
My dog (30lbs) once ate a whole pack of Cadbury chocolate buttons (milk chocolate) She was lethargic, got bad diarrhea, and vomited once. I took her straight to the vet and he said to monitor her on water only for 24 hours then introduce plain boiled rice, followed by chicken and rice. She was much better even that very evening, and was begging desperately for food....so I broke the rules and am ashamed to say I let her have a couple of doggy biscuits!
She was totally well and fine the next day.
Yes chocolate is toxic for dogs, but like Margot said, it depends on the amount of chocolate in relation to their weight. I doubt one square of chocolate would cause danger to most dogs, unless they were teacup-size dogs. But it is a very bad idea to let them have it of course. But accidents can and do happen.
Dark chocolate and baking chocolate is more dangerous than milk chocolate.
I would say just keep an eye on her carefully for the next day or so, and if she shows no ill-effects she should be just fine. If she seems at all unwell, or behaves unusually, take her quickly to the vet.
Sorry no advice "in time"... so what's the answer: is your dog ok?
One piece of chocolate would not likely cause harm, but the size of the chocolate compared to the dog is important, I would think.
I believe we should not feed, or allow our dogs to eat chocolate. So, I share your concern.