Aborigines.
Persians.
Malaysians.
Indians.
Oh....
Nevermind. They don't all come from the same place.
Blacks are more diverse than it may appear, at first.
The main reasons for visual similarity is that blacks have the genes for more prominent production of melanin. This is produced in hair, skin, and the iris of the eye.
Green and blue eyes are both caused by a lack of pigmentation of the iris with the crystalline structure of the iris determining the color (exact genetic and gestation influences have not, to my knowledge, been identified). Hazel eyes are a partial pigmentation of the iris (x-inactivation - much like calico cats).
Because of our lack of a command to produce pigment to the max, other genes have a chance to become more obvious in their expression.
If you start to look more closely at the features displayed between the different ethnic groups of blacks, the diversity begins to appear - although it is more nuanced than things as obvious as blue/green eyes, red/blonde hair, etc.