Preheat oven to 400 degrees F. Lightly spray 18 muffin cups with paper cupcake liners.
Combine the flours, baking soda, cream of tarter, and salt together in a bowl and set aside. Remove a teaspoon or two of the flour and very carefully sprinkle it over the berries. Set aside.
In a separate bowl, mix the milk, vanilla, and Greek yogurt together.
Use a mixer to beat the softened butter for three minutes on medium speed until it is white and fluffy. Add to this the Truvia and beat for another three minutes on medium speed. Add the egg and egg yolk and beat for another two minutes until the mixture is airy and light.
Add half of the flour mixture into the wet with half the milk mixture and lemon zest and then fold carefully. Repeat with remaining flour and milk. Some dry spots are OK. Fold the batter as little as possible until it's fairly well mixed together.
Muddle or mash the blackberries using a fork or wooden spoon. Some full berries are fine, but you want a lot of juice. Add to this the lemon juice and stir.
Spoon the batter into cupcake liners about 1/3 full. Spoon the blackberries on top of the batter and then top with a little more batter.
Bake for 15-20 minutes until golden brown and an inserted toothpick comes out clean. Place an oven-safe bowl or cup of water inside the oven while baking to keep the muffins moist.
Recipe and photo by Brandi Koskie. Adapted from DozenFlours.com