In a medium saucepan combine blackberries, sugar, water, lemon juice, and 1/2 tsp cinnamon. Bring to a boil. Reduce heat. Simmer, uncover, for about 8 minutes or until slightly thickened, stirring frequently. Remove from heat.
In a mixing bowl, stir together flour, oats, brown sugar, 1/4 tsp cinnamon, and baking soda.
Stir in melted butter until thoroughly combined.
Set aside 1 cp of oat mixture for topping.
Press remaining oat mixture into an ungreased 9x9x2-inch pan. Bake in a 350° oven for 20-25 minutes.
Carefully spread filling on top of baked crust. Sprinkle with reserved oat mixture. Lightly press oat mixture into filling. Bake in the 350° oven for 20-25 minutes more or until topping is set. Cool in pan on a wire rack. Cut into bars.