Answer: What you describe could be environmental or the beginnings of a disease. Blueberries are susceptible to many diseases including cane canker, twig blight, twig canker, and stem blight (similar sounding and similar in overall symptoms -- dieback -- but different diseases). Most of these diseases are prevalent in spring when moisture levels are high, temps are rising, and new growth is soft. You can minimize damage by cutting off affected plant parts (in this case affected leaves). If you find additional symptoms on stems or canes, cut them from the plant and dispose of them as soon as you see symptoms.
Hope you have a bountiful harvest!
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