Just so you are aware, your thread is going to be closed due to the Migratory Bird Act. The act means it is illegal to keep any native birds without a special license
A Google search for it says "The parents bring crickets, spiders, grasshoppers, butterflies, and moth larvae, as well as berries such as raspberry, mulberry, dogwood, cherry, and honeysuckle. The diet of nestling bluebirds is high in protein, consisting of approximately 68% invertebrates and 32% berries.".
They can be fed mealworms, chopped berries (fresh, frozen) crickets, and unsalted/treated nuts/seeds. Be warned, they need to be fed hourly for three weeks.