Omigod! You totally freaked me out when I read this as I too ordered their black silk taffeta. When I got home from Vegas I had a mini-stroke and tore open the box only to find...black silk taffeta. Mine has no slubs, has a nice sheen and everything else. Go fig.
I wonder if they weren't paying attention and sent you dupioni by accident. Either way if you aren't happy with it - send it back! Take if from a person who has "lived" with a purchase, you won't be happy if you keep it. But Fabric.com has always been real nice to me about those things, so you should have no probs.