African ancestry and the Black label (like the Negro label) are not quite interchangeable.
Imagine this: you meet an African immigrant named Samuel Bellevue. Also, you meet somebody at school who is from Los Angeles and speaks with a different accent. These two people have different backgrounds, though their relationship with Africa is used foremost to define their identity in other people's minds. Yet the relationship is different. "Wow, you're African -- from Africa!" would be the response to one. "You are African American," would be the response to the other, even if they are a vibrant mix of African, French, Spanish, Mexican, German and English just as much. There is a difference, somehow.
Black people today are a transcontinental, intercontinental blend that has emerged from centuries of colonization, trade, slave trade and eventually post-colonial, postbellum admixture. Their unifying qualities are extensions of even one drop attitudes used to enforce segregation: any visually apparent African heritage automatically qualifies somebody as Black. There is no half black, except when clarifying that somebody has one non-Black parent, because there is no half scrutiny, half segregation, or half discrimination. Black is black. Any other genetic heritage, even if it constitutes a large majority, is only an interesting footnote of being Black first and foremost.