Preheat oven to 375 degrees F. In a mixing bowl, combine room temperature butter (left out about 20-30 min), cream cheese, and sugar. With an electric mixer, beat on medium speed till smooth and combined. On low speed, add eggs one at a time until just combined. Add 1 teaspoon of vanilla.
In a separate mixing bowl, combine flour, baking powder, baking soda, and salt. Alternate adding flour and milk into the butter mixture while mixing on low speed till combined. Fold in chopped blackberries with a spatula.
Prepare a muffin tin with paper cups and spoon the mixture into each cup, filling to the top. Add a pinch of turbinado sugar to the tops (if using for extra crunch). Bake for 23-25 minutes, until light brown on top and a toothpick inserted in the center comes out clean. (If using frozen blackberries, you may need to bake a few minutes longer)
Leave the muffins in the tin for 3-5 minutes and then remove to a cooling rack.
Notes
You can defrost them a few ways, but the easiest is just to microwave for about 15 seconds. Another option is to leave at room temperature for 20 minutes.
This recipe is lower in sugar than most and relies on the sweetness of berries. If yours are super tart, you may want to add another 1/4-1/2 cup of sugar.
Another great way to warm them (without them being kinda sticky) is to place them in the oven while it preheats to 350 degrees. I find this gets them nice and warm without the stickiness on top. You can start by keeping them wrapped in foil and then open the foil the last few minutes. If you start from frozen, the time may increase.
These muffins can be stored in a freezer safe bag for up to 3 months. See post for other storage options.