For the blackberry ice cream, cut the bananas into pieces and put them in the freezer for about 5 hours. Also put the blackberries in the freezer or use frozen ones.
Put the frozen bananas in a blender/ice crusher with the berries and puree. If the fruit is very ripe, you no longer need an additional sweetener.
Otherwise, you can sweeten with 1-2 tablespoons of agave syrup or rice syrup. Serve the blackberry ice cream immediately and enjoy.