The leaf reddening is most likely due to a nutrient deficiency. For optimum blueberry production, the soil pH should be between 4.5 and 5.2 in order for several necessary micro-nutrients to be soluble in the soil-water solution and available to the plant. If the soil is too alkaline (sweet), magnesium will become unavailable to the plant, causing interveinal reddening on the leaves.
Cold spring weather can cause leaf reddening, but that disappears as temperatures warm. A lack of nitrogen can also cause leaf reddening.
You should get the soil in your blueberry patch tested for pH and nutrient values. Contact your local Cooperative Extension System office and inquire about their soil testing services.
You would also benefit from reading about home production of blueberries. Refer to: Growing Highbush Blueberries in Kentucky.
