Preheat oven to 425 degrees F + Grease a 6 jumbo muffin tin or 12 small cupcake tin

For oat crumble topping: Mix flour, brown sugar, oats, cinnamon, and salt in a small bowl. Add melted butter and blend with a fork. Mixture should be sandy and not creamy. Set aside for topping with muffins. 

For muffins: In a large bowl whisk together flour, baking powder, cinnamon, and salt until well blended. In a medium sized bowl whisk together sugar, oil, eggs, vanilla extract and almond milk. Add wet mixture into dry mixture in three separate increments. Be careful not to over mix. Add in blackberries and rhubarb to batter and mix once more or until just incorporated. Pour batter evenly into cupcake tins and top with a small handful (1/4 cup) of crumble topping to each muffin.

Bake for 5 minutes at 425 degrees and then reduce temp to 350 degrees. Bake for an additional 20-25 minutes or until the tops are golden brown and a toothpick comes out cleanly.