Should Thurgood Marshall's replacement on the Supreme Court be a black? Of course. This has nothing to do with quotas or affirmative action. It has to do with the Supreme Court's ability to deal with one of the most vexing and enduring problem areas of American law. That is race (not ethnic, not sex) relations.
The Constitution of 1787 deals with race (Article I, Section IX), and so do the Reconstruction amendments (XIII, XIV and XV), and so does the modern-day amendment (XXIV) abolishing state poll taxes. There are tens of thousands of local, state and federal civil rights laws that deal primarily if not exclusively with black-white legal status. Every term, the Supreme Court considers a number of civil rights cases that involve some of those laws.
Racism and the legal effort to end it are part of the memory, mind, blood, bone and sinew of the American experience -- we fought a civil war over it -- and of American law. Of course the Supreme Court, which decides things as a group, in face-to-face conversations and exchanges of written arguments -- needs a justice whose perspective is different, whose judgment, temperament and experience are rooted in being black.