Brown haired girls with blue eyes isn't usually considered "rare" for a few reasons -
°° Some girl born with, we'll say, blonde hair dyed her hair brown and she had blue eyes. This isn't always rare, because anyone could dye there hair and still have blue eyes
°° Everyone is different - You could have like, red hair and purple eyes, or black hair with yellow eyes. You can pretty much be a person with any hair color and any eye color. Nothing really goes together.
Brown eyes are proven to be the most common eye color, as it consists of 55% of the worlds population. Blue eyes rank 3rd place in most common, making them fairly regular. About 8% of humans have blue eyes, which is a pretty fair. In that amount, brown hair is pretty rare to be mixed with the blue eyes. Therefore, making it somewhat rare. As people with blue eyes usually end up with blonde hair. Again, its possible to have that combination.
