Never mind re: pics of the black sheath. I think I found it. It's a v-neck, right?
I think maybe the v-neck is what makes the style a bit more casual than another neckline would. HOWEVER, that neckline is crying out for a big, blingy necklace to fill it in. Do you have anything like that, just to try? I think the right jewelry (big blingy necklace) and a festive shoe would make a big difference in how dressy/casual the sheath looks.
And here's a friendly reminder. This is what friends with large accessory collections are for. If you have a buddy nearby who likes bling, and if you don't have the right item, it is perfectly acceptable to borrow such an item for a special occasion such as this. Do you like hair accessories? Something sparkly and fabulous in your hair might up the dressiness factor, too. So long as it doesn't compete with the necklace.