Preheat oven to 375 degrees F. Line a 12-cup muffin tin with liners and lightly spray with nonstick cooking spray. Set aside.
In a large mixing bowl prepare the batter by whisking together the butter, apple sauce, milk, eggs and orange zest. Beat well for 2-3 minutes until fully incorporated. Whisk in the sugar until combined.
In a separate bowl mix together the flour, baking powder and salt.
Using a rubber spatula gently gently mix together the dry with the wet ingredients until just mixed- no not overmix. Fold in the blackberries and chopped chocolate.
Scoop the muffin batter into the prepared muffin tin, dividing the batter evenly between the 12 cups. Transfer the muffin tin to to oven and bake for approximately 22-25 minutes, or until tops of the muffins are slightly golden and toothpick inserted comes out clean.
Remove tin from the oven and allow muffins to cool for at least 5-10 minutes before serving. Store leftover muffins in an airtight container or ziplock bag for 2-3 days.