The Dred Scott case was not about any "ethnicity" en masse, but about a particular individual who was legally a slave at point A, but (perhaps) not at point B. Every state had free people of African descent.
From Wackypedia (grabbed for expediency):
On March 6, 1857, Chief Justice Roger B. Taney delivered the majority opinion. Taney ruled that:
Any person descended from Africans, whether slave or free, is not a citizen of the United States, according to the Constitution.
The Ordinance of 1787 could not confer either freedom or citizenship within the Northwest Territory to non-white individuals.
The provisions of the Act of 1820, known as the Missouri Compromise, were voided as a legislative act, since the act exceeded the powers of Congress, insofar as it attempted to exclude slavery and impart freedom and citizenship to non-white persons in the northern part of the Louisiana Purchase.[13]
The Court had ruled that African Americans had no claim to freedom or citizenship. Since they were not citizens, they did not possess the legal standing to bring suit in a federal court. As slaves were private property, Congress did not have the power to regulate slavery in the territories and could not revoke a slave owner’s rights based on where he lived.
Worst SCOTUS decision ever.