From the dissent: "The need for restraint in administering the strong medicine of substantive due process is a lesson this Court has learned the hard way. The Court first applied substantive due process to strike down a statute in Dred Scott v. Sandford, 19 How. 393 (1857). There the Court invalidated the Missouri Compromise on the ground that legislation restricting the institution of slavery violated the implied rights of slaveholders. The Court relied on its own conception of liberty and property in doing so. It asserted that “an act of Congress which deprives a citizen of the United States of his liberty or property, merely because he came himself or brought his property into a particular Territory of the United States . . . could hardly be dignified with the name of due process of law.” Id., at 450. In a dissent that has outlasted the majority opinion, Justice Curtis explained that when the “fixed rules which govern the interpretation of laws [are] abandoned, and the theoretical 12 OBERGEFELL v. HODGES ROBERTS, C. J., dissenting opinions of individuals are allowed to control” the Constitution’s meaning, “we have no longer a Constitution; we are under the government of individual men, who for the time being have power to declare what the Constitution is, according to their own views of what it ought to mean.” Id., at 621."
Now, there were a number of other pro-slavery decisions by the supreme court up until Dred Scott. The difference was that Scott had been living in free territories where slavery was illegal at the state and federal level. Thomas means that, because it was specifically mentioned in state case law and federal law, the court overstepped its bounds in the decision to refuse him his rights as a citizen.
The subtext is that Thomas believes the prior rulings by the court were constitutional because the Constitution allowed for slavery at the time. He also says that "Dred Scott’s holding was overruled on the battlefields of the Civil War and by constitutional amendment after Appomattox." His belief is that the Constitution is meant to only be changed by the legislature and, regardless of what should or should not be, the court can only uphold what is specifically in the Constitution. To take that further, there are understood civil rights that are not specifically mentioned but are defined as being deeply rooted in US history and tradition. Thomas makes this case for religious liberty on pg 14-16 of his dissent with regards to gay marriage. The same arguments successfully defended slavery until it was made illegal by constitutional amendment.
Regardless of his opinions on the morality of slavery, that is a defense of the constitutionality of the practice.
As for "black face" being racist, the context is that whites dressed as blacks and perpetuated insane stereotypes for white entertainment. That's a super racist thing to do. Saying that Thomas is in black face is actually more accurate than Chris calling him Uncle Thomas in that he does not act in a subserviant manner, but in one that downplays the plight of his own people. His dissent is actually very un-Uncle Tom-like.
