Coat the insert of a 5- to 7-quart slow cooker with nonstick cooking spray. Spread the blueberries over the bottom of the slow-cooker insert.
Beat the egg whites in a large mixing bowl until soft peaks form, and set aside.
Whisk the egg yolks in another mixing bowl. Add the zest and juice, butter, and milk and whisk until blended.
Stir together the sugar, flour, and salt in another bowl and add to the egg yolk mixture. Beat until smooth, then fold into the reserved egg whites.
Transfer the batter to the slow cooker. Cover and cook on high for 2½ hours.
Allow the cake to cool slightly before serving.