Drain and rinse black bean well. Combine beans, eggs, oil, cocoa powder, salt, vanilla extract, and sugar in a blender. Blend until smooth. Fold in chopped pecans by hand.
Pour the mixture into a prepared 9x9 baking dish. Bake at 350 degrees in the preheated oven until the top is dry and the edges start to pull away from the sides of the pan, about 30 minutes. Sprinkle with powder sugar as desired.