This recipe calls for either raspberries, blackberries, or a combination of both. Any similarly-structured aggregate fruit would also work well, such as wineberries, tayberries, marionberries, boysenberries, or loganberries. While strawberries and/or blueberries would add a wonderful flavor to butter, they won't work quite as well in this recipe. Here we are adding the whole berries to the butter, relying on the action of the hand mixer to break them up into their small individual bits of juicy pulp (these are called drupelets). Because strawberries and blueberries don't have this same drupelet structure, they would not incorporate as well into the butter. However, if you don't mind this and would like to use strawberries and/or blueberries in your berry butter, just be sure to cut them into small pieces before adding them to the butter.