Make the fruitcake granola: Preheat the oven to 275. Crumble the cake into pea-size pieces with your hands. Chop up any large chunks of fruit or nuts. Scatter the pieces on a baking sheet and bake until golden brown and dry, 35 to 55 minutes, depending on the kind of fruitcake. Cool completely. (Leftover granola will keep in an airtight container for up to 1 week.)
Make the parfaits: Warm the blueberries in a small saucepan over medium heat, about 5 minutes. Strain, reserving the berries and juice separately. Return the juice to the pan and boil until reduced by half, about 4 minutes. Cool completely.
Divide the blueberry sauce among 4 tall parfait glasses. Add 1 cup yogurt to each and gently swirl. Top the parfaits with the fruitcake granola and blueberries.