1. Coat the inside of a 5- to 6-quart slow cooker with cooking spray. Add the blueberries, sugar, cornstarch, and cinnamon. Toss to combine. Cover and cook for 30 minutes on high.
2. When the timer goes off, start the biscuit topping. In a food processor, combine the flour, cornmeal, sugar, lemon zest, baking powder, baking soda, and salt. Cover and pulse to combine.
3. Add the butter and pulse until the butter is cut into pea-sized pieces, about 6 to 8 pulses. Remove the lid and drizzle in the buttermilk and heavy cream. Pulse again until the mixture holds together when pressed, about 6 to 8 pulses.
4. Using lightly floured hands, gently form the dough into 6 biscuits, each measuring about 2 1/2- to 3-inches wide and 1-inch tall.
5. Stir the blueberries in the slow cooker. Arrange the biscuits over top, and sprinkle them with the turbinado sugar. Lay a double layer of paper towels over the rim of the slow cooker, then cover, making sure that the paper towels are taut. Cook 2 hours more.
6. Carefully uncover and remove the paper towels. The berries should be bubbling, and the biscuits should be cooked through (they should feel firm to the touch). If not, re-cover the slow cooker (with another layer of paper towels) and cook 30-60 minutes longer.
7. Turn off the slow cooker, pull out the insert (if it comes out), and let stand at least 20 minutes before serving or up to 4 hours. Serve the cobbler with whipped cream or ice cream, if desired.