You're arguing that "Black" is an identity in the US because the people thus identified share a common history within the US, even though their ancestors originated from different regions and cultures before they were enslaved and shipped to North America. Yet in the next paragraph you argue that "White" is not a valid identity, because their ancestors originated from different regions and cultures, even though they share a common history within the US. How do you reconcile this double standard?

Edit: In case you're only paraphrasing a point of view which you don't hold yourself, it would probably be a good idea to use a style that clearly signals this.