I have an Excalibur Dehydrator and it states to dehydrate blackberries at 135 degrees. Please check your brand for the correct temperature. You can also use your oven by propping the door open with some foil to keep it ajar and setting temperature as low as possible. Keep checking your blackberries often if you use your oven, they will dry a whole lot faster. The time will always depend on the humidity of the room you are dehydrating your fruit. These took almost 20 hours in my dehydrator today.
I let the food I dehydrate dry for at least two weeks on the countertop before placing it in mason jars and sealing them with my FoodSaver. Use your blender to make blackberry powder in small portions. I only make powder for a month at a time. Please do not use your FoodSaver to seal the powder it will ruin your unit.