Answer: I think what you're describing is drought stress. If you recently planted the blueberries, they may be reacting to the stress by dropping some foliage, especially if they haven't been watered frequently, or if the planting hole is not draining well. Dry soil can cause dieback; wet soil can suffocate roots and cause dieback.
If you have not yet planted the blueberries in the ground, it's possible that the roots have overheated (nursery pots in direct sunshine during the summer months can absorb and radiate heat which can actually bake the roots).
I can only suggest that you keep the plants well watered, whether in-ground or still in the pots, and prune back the dead stems and foliage to encourage healthy new growth.
Best wishes with your blueberry plants.
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