I can confidently say these will be the best (and maybe only?) blue blueberry muffins you've ever had! And just in time for national blueberry muffin day. They are soft and fluffy, with a domed top and crispy streusel. This recipe uses frozen blueberries to make the muffins naturally blue, but that's not all they do - they help make this cookie recipe vegan, subtly sweet, and filled with blueberry flavor. A total winner in my books!