Well, Black people aren’t actually black either. African-Americans are a variety of colors, mostly shades of brown, but I have never seen someone who was the color of ebony or licorice.

there was also no such country as Germany or Italy until the 19th century, yet we still call people who came from those regions at that time Germans or Italians.

I would say that enslaved Africans living in America are Americans because it was their labor and creativity that helped build this country, and gave it it’s natural character. Granting them official citizenship was simply government belatedly acknowledging what had long been a truth.