Why do some African Americans men have a blue ring around the pupil of the eye?

It's a heredity and genetic trait. Me, My mother, my twin sister and my brother has brown eyes with a blue ring around the Iris and so did my maternal grandmother (may she rest in peace). I am 48 years old black female from Trinidad West Indies and I recently began looking into this trait on the internet when I noticed the ring a few years ago. I found that quite a few people with ancestors from the Slave Trade and People from the Caribbean Isles and Native American heritage have this same trait which is hereditary. My paternal grandmother was a native Arawak Indian of Trinidad (similar heritage to Native American Indian) who died off shortly after Columbus landed there.