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.