Reduce the heat to medium and cook, stirring often, until the cranberries pop and the mixture becomes quite juicy, 2 to 5 minutes. Continue to cook, still stirring often, until the sauce goes from very runny and juicy to syrupy and jammy, 12 to 15 minutes. (The blueberries can be made up to 5 days ahead. Let them cool, then cover with plastic wrap, pressing the plastic directly onto the blueberries, and let cool.)
