Blueberry swirl muffins: the secret ingredient to these blueberry muffins is the blueberry swirl. I threw some blueberries and sugar in a saucepan and heated it until the blueberries were soft and mushy. With a spoon, I stirred the blueberries around and mashed them until they became a thick sauce-like mixture. Once I was done with my muffin batter and divided the batter into my muffin tin, I swirled a spoonful of the blueberry mixture in each muffin and swirled it into the center with a toothpick. Lastly, I sprinkled streusel over each muffin and popped them in the oven.