Justice Elena Kagan said Timothy Foster's case seemed as clear a violation "as a court is ever going to see" of rules meant to prevent racial discrimination in jury selection.
Foster was sentenced to death in 1987.
He argues that excluding black people from the jury made his sentence more harsh.
The prosecutor in his case had asked for a death sentence to "deter other people out there in the projects".
The Supreme Court will determine whether prosecutor Stephen Lanier and his team violated the constitutional rights of Foster.
Mr Lanier has denied any intentions to discriminate against Foster.
There is still much concern that African Americans are being struck from US juries at a higher rate than whites.
A 2011 lawsuit argued that 82% of black jurors were denied in death penalty cases in Houston and Henry counties in Alabama. A 2015 study of jury strikes in Caddo Parish, Louisiana, showed that prosecutors struck black jurors at three times the rate they struck non-blacks.
Potential jury members are selected from a large pool, then whittled down to 12 members. Each trial lawyer is given a number of "peremptory strikes". If a peremptory strike is challenged as being racial in nature, the lawyer must give a so-called race-neutral reason.
This can be a low bar to clear. An Equal Justice Initiative study found a "startlingly common" reason for striking black jurors was "low intelligence". Other reasons included living in "high crime" areas, or being on food stamps.
The US's highest court had ruled in 1986 that jurors could not be excluded due to race.
In 2006, the case was re-opened when the state of Georgia made public notes that showed prosecutors had singled out black people during jury selection - the world "black" had been circled next to the "race" option.
One handwritten note said "Definite No's" with six people, five of whom were black.
Three prospective black jurors were labelled on the notes as "B#1", "B#2" and "B#3".
The US Supreme Court is determining whether racism played a role when an all-white jury put a black teenager on death row for killing a white woman.