Step 1: Combine graham cracker crumbs, melted butter, and granulated sugar in a medium bowl until well mixed. Press the mixture firmly into the bottom of a 9-inch springform pan. Place in the freezer for 15-20 minutes to set.
Step 2: While the crust chills, soften 3 cups of vanilla ice cream at room temperature for 10-15 minutes. In a separate bowl, gently fold the mango puree into the softened ice cream until just combined, creating a marbled effect.
Step 3: Spread the mango ice cream mixture evenly over the chilled graham cracker crust. Return the pan to the freezer for at least 1 hour, or until the mango layer is firm.
Step 4: Soften the remaining 3 cups of vanilla ice cream. In a separate bowl, gently fold the blueberry puree into the softened ice cream until just combined, creating a marbled effect.
Step 5: Carefully spread the blueberry ice cream mixture over the firm mango layer, smoothing the top.
Step 6: Cover the springform pan loosely with plastic wrap or aluminum foil. Freeze the mango blueberry ice cream cake for a minimum of 6-8 hours, or preferably overnight, until completely firm.
Step 7: To serve, remove the cake from the freezer and let it sit at room temperature for 10-15 minutes. Run a thin knife around the edge before releasing the springform sides. Garnish with fresh mango slices, blueberries, and mint, if desired. Slice and serve immediately.