Combine blackberries and sugar in a small saucepan and cook, over low heat, until slightly thickened and sugar is dissolved. Mash the blackberries slightly with a fork and let cool.
In a blender, combine 1/3 cup milk, 10 ounces of ice cream, vanilla bean seeds and the nutmeg and blend until smooth. Fill a tall milkshake glass 1/3 of the way with some of the milkshake, add a few tablespoons of the blackberry puree, fill 1/3 of the way with more milkshake, then a few tablespoons of the puree and finish with the remaining milkshake. Use a straw to stir and swirl the mixtures together. Garnish with a few fresh blackberries and serve.