Psst! All those black slaves that were shipped by white people to the Americas, were actually captured and sold by other blacks.
You see, Africans don't consider themselves "black". They feel themselves Hausa, Yoruba, Igbo, Ijaw, Ibibio, Kanuri, Annang, Etsako, Tiv, or Efik - those are just the main ethnic groups from Nigeria.
And people from one of those groups had no scrupules about enslaving people from any of the others.