1) Combine the blueberries, caster sugar, rum and vanilla pod seeds in a bowl, let sit at room temperature for 10 minutes. Lightly mash the mixture with a fork and let sit 10 minutes longer.
2) Put the milk and the blueberry mixture in a blender and blend until smooth. add the ice cream and blend until smooth. Divide between 2 tall glasses and serve with a straw. Garnish with a few blueberries.