Once you have your berries you can choose to separate out the seeds or leave the berries whole. If you have frozen berries you can add them whole with the bananas. If separating the seeds, you can pour about 1/4 - 3/4 cups of prepared blackberry juice into your bananas as you puree. This is also great with blueberries and strawberries.
Here is a link for my version of a non-dairy ice cream alternative: