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.
↑ ↑ ↑ Hope this helps! :D