Use any kind of berries you like in these muffins, fresh or frozen. Mix them up, or use all one kind. It's up to you. When berries are in season, I like to make these muffins with fresh berries. The rest of the year, I use frozen berries. Honestly, frozen berries are one of my favorite modern conveniences. While these somehow feel like the kind of thing I want to eat on spring and summer mornings, whipping up a batch in the dead of winter is a good antidote (albeit temporary) to the winter blues.