Grease a 9×13 inch pan and line the bottom with parchment. Preheat oven to 350°F.
Roll the dough into a 12×18 inch rectangle on a lightly floured surface.
Spread softened butter all over the dough. Sprinkle the cinnamon sugar evenly on top.
Scatter the blackberries over the sugar layer. Gently press them in.
Roll up tightly from a long side. Pinch the seam to seal. Slice into 12 rolls.
Arrange rolls cut-side up in the pan. Cover and let rise until puffy, 25–35 minutes.
Bake until lightly golden and bubbling, 24–28 minutes. Cool 10 minutes in the pan.