Call and ask your vet, or an emergency vet.
Dark chocolate is worse than milk chocolate.
From personal experience ... my RB basset hound ate a big Valentine's heart-shaped box full of chocolate once. Not just the chocolate, mind you, but the box, the ribbon, the plastic flowers ... everything. And it was a BIG box. 
I took her to the vet, of course, but she was perfectly fine, not even the slightest bit ill in any way.
"We give dogs the time we can spare, the space we can spare and the love we can spare. And in return, dogs give us their all. It's the best deal man has ever made" - M. Facklam
"We are raised to honor all the wrong explorers and discoverers - thieves planting flags, murderers carrying crosses. Let us at last praise the colonizers of dreams."- P.S. Beagle
"All that is gold does not glitter, Not all those who wander are lost; The old that is strong does not wither, Deep roots are not reached by the frost. From the ashes a fire shall be woken, A light from the shadows shall spring; Renewed shall be blade that was broken, The crownless again shall be king." - J.R.R. Tolkien
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