In a medium cast-iron Dutch oven, pour oil to fill halfway, and heat over medium heat until a deep-fry thermometer registers 350°.
In a large bowl, whisk together flour, ¼ cup sugar, ¼ teaspoon cinnamon, and salt; whisk in buttermilk, egg, and vanilla until a thick batter forms. Gently stir in blackberries.
Work in batches, carefully drop batter by ¼ cupfuls into hot oil. Fry, turning occasionally, until golden brown, 3 to 4 minutes. Remove using a slotted spoon, and let drain on paper towels. (Let oil heat to 350° between each batch.)
In a small bowl, whisk together remaining 1 cup sugar and remaining 1 teaspoon cinnamon; roll warm fritters in sugar mixture to coat. Serve immediately.
Recipe by Southern Cast Iron at https://southerncastiron.com/blackberry-fritters/