Coat a 1 1/2- or 2-quart slow cooker or a 1 1/2-quart casserole with cooking spray. If using the 1 1/2-quart casserole, place casserole inside a 5- to 6-quart slow cooker and add enough water to cooker to reach halfway up sides of casserole.
In a medium saucepan combine milk, polenta, butter, and salt. Bring to boiling over medium-high heat, whisking constantly. Boil for 2 to 3 minutes.
Transfer mixture to the prepared 1 1/2- or 2-quart slow cooker or the prepared casserole. Cover slow cooker and cook on high-heat setting for 1 1/2 to 2 hours, stirring once halfway through cooking.
In a small bowl combine mascarpone cheese and maple syrup. Stir mascarpone mixture and the 1/2 cup blueberries into hot cereal. Sprinkle individual servings with additional blueberries and, if desired, pecans.