I assume they are in the ground? What part of the country are you in? What kind of blueberries do you have? Soil ph, type of water used, type of fert used? We need info

John, I had same problem with my BB. If the root system is healthy, Tthis red leave has something to do with soil ph level too high that the BB can't take in the nutrients it need, such as Fe. Need to lower ph by addind some sulpur in the soil, or vinegar water. One is longer term solution, one is short term solution.
Another thing I found with my few BB plants is the root was rotting which can't take in nutrient either no matter what the soil ph level is.