If berries are frozen, slightly thaw. In a bowl, stir together sugar and flour. Add fruit to sugar mixture, tosing well and spread mixture into baking dish.
Preheat oven to 375.
Coarsely chop pistachios. In a bowl, mix the flour and sugars. Cut the butter into cubes with your fingers or a pastry blender, blending the flour mixture until it resembles coarse meal. Add pistachios and toss well.
Squeeze a handful of topping together and coarsely crumble in chunks over filling. Squeeze and evenly crumble remaining topping over filling in same manner.
Bake crisp in oven until filling is bubbling and topping is crisp and golden, about 50 minutes. (can be done in advance).
Just before serving, heat crisp in 300 oven for 20 - 30 minutes until just warm. Serve with ice cream or frozen yogurt.
Note: When measuring out the fruit. I just add the blueberries and enough rhubarb to the baking dish, taking in to allowance the shrinkage of cooking. If no rhubarb is available, just use all blueberries.