The Dark Blade is a weapon that is in 2011 Ninjago sets. Although the picture shows a dark metal, no dark blades exist in that color, so it's basically the same as the Silver Dark Blade.
Ninjago.com Description:
Not your typical sword, the Dark Blade actually looks for trouble. It pokes and prods life with its jagged point trying to do as much damage as it can. It’s a weapon of great fear, and it was created especially for the Skeleton Army.