I have blackberries frozen in gallon bags from my tame briars I think you can buy blackberries at grocery
Put berries on the stove with water and sugar let cook until juice is thick taste test to see if you have enough sugar in them set aside.
1 stick butter
1 and 1/2 cup Bisquick
1 cup sugar, 1 cup milk
Melt butter put in baking dish stir up Bisquick, sugar, and milk pour in the center of butter mixture the pour blackberries juice and all bake on 350 degrees until golden brown or check to see it’s done.