If Biden keeps his pledge and nominates a black woman, and if she is confirmed, then, for the first time, the percentage of black people on the Court (22.2%) will exceed the percentage in the population (12.1%).
Women will still be underrepresented in proportion to the population — 44.4%, instead of 50.5%.
Does 22.2% seem like too much representation for black people on the Court? Consider that there has never been even one Asian American or a Native American nominated to the Supreme Court. But also consider that many people believe that Clarence Thomas, because he is conservative, doesn't represent black people at all.
Look how clearly Thurgood Marshall stated that position as he vacated the seat Thomas took (scroll to 2:30):