1. Preheat the oven to 350ºF (180ºC).
2. Begin by combining blackberries with 3 teaspoons of sugar in a small bowl. Coarsely mash with the back of a fork. Set aside.
3. Arrange a fine mesh sieve over a mixing bowl. Add the all-purpose flour, coconut flour, xanthan gum, baking powder and sea salt. Gently tap the side of the sieve until all of the dry ingredients have passed through into the mixing bowl.
4. In a large mixing bowl whisk together the wet ingredients including the canola oil, granulated sugar, vanilla and orange juice. Crack in one egg at a time making sure to beat in between each addition. Drizzle the milk into the wet ingredients while whisking until completely incorporated.
5. Mix the dry ingredients into the wet ingredients until just combined, do not overmix.
6. Line a loaf tin with tin foil or parchment paper. Spray or grease the inside to avoid sticking.
7. Add half the batter in the loaf tin. Top with blackberry mixture and top with remaining batter.
8. Using a knife insert it into the batter and carefully draw "S" shapes down the length of the pound cake.
9. Bake for 30 to 45 minutes until a toothpick comes out clean and the top of the cake is golden brown.
10. Let the cake cool in the loaf tin for 15 minutes and then transfer to a rack to finish cooling.