Prepare. Prep your slow cooker as desired, Nonstick spray or liners can work.
Bottom Layer. In a medium bowl, use a whisk to combine together the brown sugar, cinnamon and melted butter until it takes on a 'sandy' appearance. Then evenly spread HALF this mixture over the bottom of the slow cooker.
Bread. Over the brown sugar layer, place the bread so that it covers the bottom of the slow cooker. You can cut the bread as needed to fill in any gaps.
Blueberries. Sprinkle about a tablespoon of the remaining brown sugar mixture over the bread. Then top this with a half cup of the blueberries and then a layer of bread slices. Repeat this step until all of the berries have been used.
Custard. To make the custard layer, whisk together the eggs, milk and vanilla in a large bowl until well blended. Then pour this mixture over the the ingredients in the slow cooker so that it can soak into the bread slices.
Chill. Cover and refrigerate your blueberry casserole or up to overnight.
Slow Cook. Slow cook your casserole on low until done. Your casserole is done when the bread is tender and the custard layer is set (three to four hours, depending on the size of your slow cooker).
Serve. When your casserole is done slow cooking, let it set for about 15 minutes before serving. This will help make it easier to serve.