Mix the maple syrup and blueberries in a small, heavy-bottomed pan.
Bring them to a boil, and cook for only one minute or two minutes.
Remove the pan from the heat, smash the berries up using a fork, and allow the mixture to cool down to room temperature.
Once it is cool, process the mixture and the unsalted butter at room temperature in a food processor for a few minutes to achieve a more buttery consistence. Scrap the edges down as needed during this process.
Put the maple-blueberry butter on your Belgian waffles, and serve.