It’s June, not February, but that hasn’t stopped Clarence Thomas from offering some lessons in black history to his colleagues on the Supreme Court.

The occasion is an Indiana abortion case, Box v. Planned Parenthood. In 2016 the state passed a law banning so-called selective abortions—based on race, gender or disability—and requiring that a baby’s remains be cremated or buried after the procedure is finished. The law was challenged, and the Seventh U.S. Circuit Court of Appeals found both provisions unconstitutional.

To...