This recipe works with other berries as well. Besides blueberries, you can also swap in 1 cup of raspberries or strawberries.
Peel and slice bananas and place in a gallon-size ziplock bag. Squeeze out air. Place flat in freezer and freeze until firm, at least 6 hours and up to overnight.
In a food processor, puree bananas, 2 Tbsp. agave and 1/4 cup almond milk, scraping sides of bowl, until smooth, 2 to 4 minutes. Spread ice cream in an 8-inch square freezer-safe container, cover and place in freezer.
Clean out food processor bowl. Add blueberries and remaining 2 Tbsp. agave and 1/4 cup almond milk and process until smooth, about 1 minute.
Remove banana ice cream from freezer and dollop with blueberry mixture. Using a rubber spatula, gently swirl together. Cover and freeze until slightly firm, about 30 minutes. Serve immediately, or store in freezer in an airtight container for up to 3 days. Let soften slightly before scooping.