Preheat oven to 350F, place an oven rack in the center of the oven. Grease a 10 cup bundt cake with butter and dust with flour. Set a side.
Using a fork, mash the blackberries to a fine puree. Place the puree in a sieve over a bowl and set a side.
Sift together the flour, baking powder, baking soda and potatore starch. Stir to incoporate and set a side.
In a large measuring cup, mix together the sour cream and lemon juice. Set a side.
In a bowl of a standing mixer, place the sugar, butter and lemon zest and beat on medium speed until fluffy and light color. About 4-5 minutes.
Lower speed to low and add eggs one at a time, waiting for each egg to completely incoporate before adding the next.
Add the flour in three additions alternating with the sour cream mix. Starting snd finishing with the flour mix.
Once you added the last portion of the flour mix, turn mixer off and using a rubber spatula incoporate any flour/egg strides.
Scoop about 1/3 of the batter into a medium size clean bowl and fold in the blackberry purre.
Using regular size cookie spoons, spoon the batters into the greased pan in alternating patterns (white batter next to blackberry batter).
Bake for 45-50 minutes or just until a skewer comes out clean from the center of the cake.
Remove from the oven and allow to completely cool.
Place the freezed dried berries in a coffee grainer (food processor is ok) and grined to a fine powder.
Add the sugar and grind to your desire texture.