Black eyes, referring to the appearance of the iris as predominantly black or extremely dark brown, are rare in people because of the way eye pigmentation works. The color of the iris, which determines eye color, is determined by the amount and distribution of a pigment called melanin.
The two primary types of melanin involved in eye color are eumelanin, which is brown to black, and pheomelanin, which is yellow to red. The combination and concentration of these pigments determine the variations in eye color, ranging from light blue or green to shades of brown.
The rarity of black eyes can be attributed to the relatively lower concentration of eumelanin in the iris of most individuals. Even in people with dark brown eyes, the eumelanin concentration may not be high enough to appear truly black. Instead, light is absorbed by the iris, resulting in a brown appearance.
It's worth noting that eye color is a complex trait influenced by multiple genetic factors. The genetics underlying eye color inheritance are still being studied, and variations can occur due to the interplay of various genes and alleles.
While black eyes are generally rare in people, there are exceptions, such as certain populations with higher eumelanin concentrations, such as some individuals of African, Asian, or Indigenous descent. Additionally, certain medical conditions or medications can cause temporary or permanent darkening of the iris, resulting in a darker appearance of the eyes.
Overall, the rarity of black eyes in people is primarily due to the distribution and concentration of melanin pigments in the iris, which typically results in a range of eye colors rather than pure black.