Place the first 5 ingredients in a small pot and whisk until completely smooth. Mix in the rhubarb. Bring to a simmer, and then very gently simmer until the rhubarb is just tender, but still holding its shape, about 5 minutes. Mix in the blueberries, and then remove from the heat and cool to room temperature. Transfer compote to a serving bowl, cover and refrigerate until needed. To serve, slice the pound cake into 12 slices and set two on each of six dessert plates. Spoon some of the compote alongside, garnish with mint sprigs, if using, and enjoy.