Origin of the word Zenji which used to refer black people in Middle East and Turkey is related to Sun. Ginger also derives from it (Pronounced like Zenjefil in Turkish). It is also geographical name for African content. Like Zanj and Sea of Zanj. English word for the Sun also probably of same origin. Zenci possibly relates to blackening of vision after exposure to direct strong Sun light. It can be speculated that people were thinking it was similar to getting darker by exposure to Sun. Especially considering Arabs are not in-born blacks. But they were getting really dark depending on the sun exposure.
Although connection with Sun and this word is clear. Exact derivation is not.