I live in Zone 7 and have 11 blueberry bushes, North Sky,Top Hat, Patriot, Blue Crop, Sunshine and one or two other varities. The berries will appear to be ripe but they don't have the blueberry flavor. They produce well but not a blueberry flavor. Do they need more fertilizer? I use Jobes Organics berry fertilizer and Blueberry Success soil from Gurneys Nurseries.
Certified GKH Gardening Expert
Sometimes, it can help your blueberry plant to apply a little iron sulfate, and even molasses can help feed your microbiome, which will in turn feed your plants to be sweeter. This collection of articles will help you to properly care for blueberries: https://www.gardeningknowhow.com/edible/fruits/blueberries/
Growing in acidic soil and fertilizing yearly will help with Blueberry flavor.