When my colony mealworms grow to a size comparable to those I received originally I remove them from the bran pail, and put them in plastic containers to keep in the refrigerator, where they will remain in the larval stage. You can freeze some mealworms, which winter birds will eat. But DO NOT supply frozen worms when adults are feeding young nestlings or fledglings. While special feeders are available, I use a small tin or plastic container near where the bluebirds are active. You may want to increase this when young birds are being fed in the nestbox.
Mealworms are a perfect lure for photography and close-up pictures.
Many excellent bluebird pictures attest to this. Others may have variations on raising mealworms. Whatever method works, mealworm feeding adds to the joy of bluebirding, and can have a positive impact on nesting success.