In a medium bowl, stir together the brown sugar, cinnamon, and melted butter until completely blended and sandy in appearance.

Prepare a slow cooker crock with nonstick cooking spray. Sprinkle the bottom of the slow cooker with ¼ of the brown sugar mixture.

Over the brown sugar, place an even layer of the sliced French bread. Cut pieces of bread if needed to fill the gaps.

Over the bread, sprinkle ¼ of the brown sugar. Top the brown sugar with ¼ of the blueberries. Repeat the layers twice, finishing with a layer of blueberries.

In a large bowl, whisk together the eggs, milk, vanilla, and salt, beating until well blended.

Pour the egg mixture evenly and slowly over the bread, allowing it to soak into the bread.

Cover and refrigerate the casserole for at least 1 hour or up to overnight.

Place the slow cooker crock into the slow cooker and cook on low for 3 to 4 hours. Allow the slow cooker to rest for 15 minutes prior to serving.

Serve the French toast with whipped cream, powdered sugar, syrup, and/or additional blueberries.