See, Louisiana v. United States, 380 U.S. 145 at 151-52 (1965), in which the court found that interpretation tests, such as Louisiana's requirement that an applicant give a reasonable interpretation of any section of the state or federal constitution, were adopted for the frank purpose of disfranchising Negroes, it being understood that the registration officers would use their discretion for that purpose. And, when the United States attorney general succeeded in having a state literacy statute declared unconstitutional, another slightly different one would be enacted to take its place.
Why should one evil act of the state justify a greater evil? Why does the disenfranchising of blacks justify the freeing of a criminal who may go off and kill other blacks? Why should the public suffer just because the state made a mistake? That makes no sense.
Yes there were literacy tests, I took one in 1960 and it was very easy, you read like three word simple sentences and it took 1 minute to do. And blacks were voting, my precinct was overwhelmingly black.
A literacy test did not result in disenfranchisement of anyone but the totally illiterate. So blacks were a considerable part of the voting population. A handful were disenfranchised along with a handful of dumb whites. I don't think that is a bad thing myself.