I use cocoa powder when I make chili. Last month while I was making it I had the jar of powder on my kitchen island within reach of my Beagle. She jumped up, knocked it on the floor, and grabbed a big mouthful of it. I scolded her, cleaned it up, and didn’t plan to give it much more thought because it didn’t look like she got much.
Not long after, she started throwing up and shaking uncontrollably. She wouldn’t leave my side and was clearly in distress. I found this site: http://www.petmd.com/dog/chocolate-toxicity at Pet MD which showed that just 1 oz (2 TB) of cocoa powder was a severe dose for a dog her size. So, off to the emergency vet hospital we went.
They induced vomiting, gave her an IV and medications, and had her spend the night to be monitored. She ended up fine in the end, but it was sure scary along the way.
So yes, if you look at that website I linked above you can see that many chocolates aren’t very dangerous, but don’t assume that chocolate isn’t dangerous. Dark and unprocessed is very dangerous and can quickly kill a dog.