It appears to be on the lowest branches. There is a spore bore disease that lies dormant in the ground until the spring rains come and then is released up to the plant causing brown spots and premature turning of color. From the picture it is not possible to tell for sure. Several things: always remove all the leaves after they fall every year, mulch is acid containing mulch around the plant in very early spring before the rains come, make sure you ph is low enough, maybe more sulfur. I can't tell the growing habits and type (southern low bush, northern high bush etc) from the picture. In all but the smallest types of blueberries those really low lying not upright branches are pruned off. They are too close to the ground therefore getting wet and staying that way to long. That promotes fungal diseases as well. Therefore I would prune off those branches and destroy them. Then mulch around it and hope you caught is soon enough.