Combine half of beets, half of reserved beet liquid and half of orange juice in blender. Blend until smooth. Blend in half of buttermilk and 1 1/2 tablespoons chopped dill. Transfer to large bowl. Repeat with remaining beets, beet liquid, orange juice, buttermilk and 1 1/2 tablespoons dill. Season with salt and pepper. Chill at least 3 hours. (Can be made 2 days ahead. Cover and keep chilled.)
Garnish soup with cucumber and additional dill. Ladle into bowls.