I have black berries frozen in gallon bags of cranberry produce that I use and think you can buy black berries at the grocery storePut the berries on the stove with water and sugar and cook them until the juice thickens and test the taste to see if you have enough sugar left.
1 stick butter°
1 & 1/2 C Bisquick
° 1 cup of sugar, 1 cup of milk
Melt the butter in a baking dish, mix the bisquick, sugar and milk in the center of the butter mixture, pour in the blackberry juice and cook at 350 degrees until golden or cooked.