Diane Kruger, a German actress and fashion model, is most well known for her role as Bridget von Hammersmark in "Ingenious Bastards" and her portrayal of Helen in the movie "Troy". Kruger also studied ballet at the Royal Ballet academy before an injury ended her dancing career.
The costume designer for "National Treasure" was Judianna Makovsky. The dress was indeed by Judianna Makovsky and is actually blue not black. You can read more about it here: http://clothesonfilm.com/national-treasure-diane-kruger-a-dress-for-action/5826/