How have you been watering it and is it in well-draining soil? Over or under-watering could cause the leaf drop/browning you are seeing. I would strive to keep the soil lightly moist (not damp/soggy or dry).
Another reason could be rust which, in extreme cases, can cause leaves to brown and drop. More information can be found here: Cornell University PDF on blueberry rust:
The proper pH for blueberry plants is actually between 4 and 5. Here is an article on blueberry soil pH that will be useful:
For more information on the care of blueberry bushes, please visit the following link: