On the plant that seems to be affected on only that bottom smaller branch so far, I would cut if off and destroy it. That might slow the process on that plant as well. I am also wondering about heat and water stress. If the plant gets too hot and/or dehydrated during the heat the leaves will sometimes turn fall colors and start dropping. When that happens this early in the season you will lose at least the most affected branches if not the whole bush. So, which way is that wall facing if its is east or worse yet south you may be frying the bushes. Blueberries need a lot more water than most people think and for far longer during the season then most people realized. If the ph of the soil is not acidic enough that will also add to the stress. If you are not giving the plants sulfur at least 3 times during the growing season or used fertilizer not recommended for acid loving plants like azaleas then that could also be a factor. So, minimum increase the water especially during hot spells and correct if needed the fertilizer/sulfur issue. If these are against east or south facing walls then rigging up some sort of shade during the heat of day may also help. Please read the article above as well. The Oregon State Extension has many good articles on blueberry growing.