Sounds like your blueberries have "Mummy Berry." This happens when a rusty growth occurs on leaves and stems and the berries cannot take up nutrients. An0ther cause is lack of nutrients because your soil is not acid enough or too dry. Blueberry plants need lots of organic matter in the soil and a low pH of between 4 to 5. You did not mention where you live but I suggest you get soil test to determine if you need to add peat moss, pine needles or a commercial soil additive to make the soil more acid. One shortcut is to use a Rhododendron or Azalea fertilizer around your blueberries to make the soil more acid and the plants more productive. Making the soil more acid allows the plants to access the nutrients that are locked up in your soil.  Keeping the soil moist is another way to make nutrients available to the plants and the berries plump. Keep growing, Marianne Binetti