Is it absolutely true that Hugo Black was a Klansman?
Yes.
wikipedia:
In 1921, Black defended E. R. Stephenson in the sensationalistic trial for the murder of Fr. James E. Coyle. He joined the Ku Klux Klan shortly after, thinking it necessary for his political career. Running for the Senate as the “people’s” candidate, Black believed he needed the votes of Klan members. Black would near the end of his life admit that joining the Klan was a mistake, but said “I would have joined any group if it helped get me votes.”[9]