I just looked it up on news.google.com\archivesearch (this shows all old newspapers from the Century), and sure enough. It’s true. Lots of newspapers in the 1930’s talk his Klan membership.
Hugo Black’s prior membership in the Klan was known, but the Democrats hid the actual evidence until after his confirmation.
Black left the KKK formally only for expedience, but he never really left it intellectually. What attracted Black to the Klan was its anti-Catholic positions, including the KKK doctrines of supporting government schools and the “sacred principle” of “separation of church and state”. Indeed, Black was well known in the KKK for his “eloquent” anti-Catholic speeches.
These two points were, in fact, an explicit part of the KKK membership oath, which Black took and may have administered to others. The reason for these two KKK doctrines was that they were directed at suppressing Catholicism. Government schools were seen as a way of coercively protestantizing Catholic children, and the separation doctrine was really just code for separating Catholics from their tax money by making sure that all education money flowed to government schools. This is why we had thirty years and more of litigation at the constitutional level over “aid to parochial schools”.
Black, not surprisingly, authored the Everson opinion in 1947. Everson for the first time applied the Establishment Clause to the states and inserted Black’s favorite KKK doctrine into our constitutional jurisprudence - “separation of church and state.” It only took 15 years for Black’s liberal, secular humanist allies to hoist Protestants on their own petard by driving Christianity out of government schools altogether using Black’s KKK separation doctrine.
Government schools and their financing arrangements - all courtesy of the KKK and its nativist predecessors. I certainly hope that all “conservatives” addicted to aid-to-dependent-parents are proud of the remarkable intellectual heritage of “their” really special government schools.