Prepare a cooking pan and put in the blackberries, strawberries, and stevia blend. Place on top of the stove and cook for about 5 minutes until a syrup is produced on the bottom of the pan and the fruits start to soften.
Remove from the heat and leave to cool for a few minutes. Once the mixture cools down completely, transfer to a blender and blend until it smoothens. Set aside.
Beat the whipping cream and vanilla extract together using a mixer. Beat for about 2 minutes until the mixture firms.
Pour the blended fruits to the whipping cream mixture and fold gently using a spatula. Combine the two mixtures thoroughly.
Transfer the mixture to some popsicle molds. Freeze for at least 3 hours.
When the popsicles are frozen, remove from the molds and serve cold. You can keep them in the freezer for a couple of months and enjoy anytime.