If using fresh blueberries, rinse thoroughly and drain well. If using frozen blueberries, thaw in a fine sieve.
Combine blueberries, brown rock sugar, vodka, pepper berries, vanilla pod, star anise, cinnamon stick, and ginger slices in a clean 2-liter glass jar with an airtight rubber seal.
Place the jar in a cool place for 2 months. Shake the jar once per week.
After 2 months, strain the liqueur through a fine sieve into a clean bottle. Discard the berries and spices.
Store the bottled liqueur in a cool place for another 2 months to mature.
Optional - To make the liqueur milder: Heat 50 g sugar and 50 g water in a small saucepan over medium heat, stirring constantly until the sugar dissolves and becomes syrupy, about 3-5 minutes. Cool the syrup completely, then add to the liqueur to taste. Let mature for 1 month.