Preheat the oven to 375°F. Lightly butter a 12 cup standard muffin tin or line with muffin wrappers.
In a medium size bowl, stir together the flours, brown sugar, baking powder, and salt.
1 cup (125 grams or 4.4 ounces) bleached, all-purpose flour, ¾ cup (80 grams or 2.8 ounces) whole wheat flour, ½ cup (110 grams or 3.9 ounces) brown sugar, 1½ teaspoons baking powder, ½ teaspoon salt
Add the cooked oatmeal and milk to the bowl of a stand mixer or a medium to large sized bowl if using a hand mixer or combining by hand. Mix on medium speed for 2 to 3 minutes.
½ cup milk, 1 cup cooked oatmeal
Add the melted butter and mix until combined.
½ cup (113 grams or 4 ounces) unsalted butter, melted
Add the eggs one at a time, and beat on medium speed for one minute after each addition until combined. Scrape down the bowl as necessary.
2 eggs
Add the orange liqueur or vanilla and beat for one minute.
1 teaspoon Grand Marnier or other orange liqueur or vanilla
Add the dry ingredients and mix on low or by hand until the dry ingredients have been moistened.
Add the chopped blackberries and stir by hand gently to combine.
1 cup blackberries, chopped in half or quarters if larger
Use a large cookie scoop or ¼ cup measuring cup to scoop the batter into the muffin tins.
Bake for about 20 minutes, until a toothpick inserted into the batter part of the muffin (i.e. not through a blackberry) comes out clean.
Cool in the tin for 10 minutes and then turn out onto a cooling rack to cool completely.
Muffins can be stored in an airtight container at room temperature for 3 to 4 days. For longer than that, muffins can be frozen for 3 to 6 months.