In a powerful blender or processor add in the chopped bananas. Addin the blackberries and vanilla. Processuntil combined.
You will need to use a tamper to ensure it blends evenly or a silicone spatula to scrape the sides.
This can take anywhere from 5-15 minutes.
Enjoy immediately as soft-serve ice cream (or place into a lined dish and freeze for thirty minutes.). Or, place in the freezer for 4-5 hours for a firmer ice cream consistency.
Video
Notes
To make it easier on your blender, chop the frozen bananas into chunks before using them (they don't freeze totally solid).
Do not add liquid to make the process faster. This will ruin the consistency. Be patient, scrape down the sides frequently, and keep going. Depending on your machine, it can take anywhere from 3-10 minutes of scraping and blending.