Eh, they're black because their ancestors were from sub-Saharan Africa in the relatively recent (in evolutionary terms) past. Even people with equally dark skin aren't colloquially considered black if they don't have African ancestry

But you're right that American blacks were both a specific subset of Africans, and have been separated just about long enough to have developed a slightly distinct distribution of hereditary traits, albeit still closer to modern day Africans than anyone else. So could be considered separately if you're getting into the fine technical details.

But I wouldn't consider "dumber than a bird" to be small detail.