Blueberries are naturally acidic but it's not the acid your're thinking of. It's an acid like citrus fruits (lemon, lime, grapefruit, orange, etc.). Their makeup contains many acids but these acids are not acidic. In other words, eating them won't create MUCUS but will bust up mucus like the lemonade fast which utilizes lemons and cayenne pepper (Capsicum fruit). Eating blueberries WON'T create mucus in your body but will greatly help yur body. Remember, it is not wise to eat all alkaline foods. There must be a balance, that's why it's called the "pH" BALANCE, so some acid foods are necessary; the ratio is something like 80% alkaline to 20% acid. 20% acid does not mean 20% ACIDIC, i.e. meat, dairy, processed junk food, eggs, etc.
Many fruits like citrus just contain a lot of acid compounds in them which dominates their makeup. Nature herself does this for her balance and she too is familiar with the pH balance. Many nuts too are acid in nature but eating a lot of them won't lower you pH balance. The threat to human health is COOKED FOOD!
Whether the food is alkaline or acid, just let it be eaten RAW and you will benefit healthwise.