Preheat oven to 375°. In a bowl, mix sugar, cornstarch and salt. Add blueberries and rhubarb; toss to coat. Transfer to a greased 8-in. square baking dish.
For topping, in a small bowl, mix baking mix, sugar and salt. Cut in butter until crumbly; stir in oats and almonds. Sprinkle over filling. Bake until filling is bubbly and topping is golden brown, 40-45 minutes.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.