And I'd have to say that I disagree completely that the Voting Rights Act is constitutional. More broadly, the power to enforce a law does not entail a power to add new meaning to the law. It means only the power to take appropriate action against violators. Either the 15th amendment prohibits literacy tests or it doesn't. If it does, then Congress would have the power to decide what to do with state officials that impose them. But of course, it doesn't mean that. The 15th amendment is one of the clearest provisions in the Constitution, scarcely requiring any "interpretation" at all. I mean, talk about screwin' up a free lunch.
These amendments were not drafted by amateurs. They were drafted by professional politicians who understood how how courts and legislatures actually work. They felt that the Fifteenth Amendment was required because they really wanted black citizens to be able to vote. And they knew that legislators in some of the states would do all that they could to prevent the enfranchisement of blacks, no matter what the Fifteenth Amendment might say.
A state doesn't have to expressly prohibit blacks from voting in order to prevent or discourage them from voting. There's lots of other ways that are quite effective. A state might decide to just not have any polling places within black communities and require that blacks who wish to vote travel to neighborboods that are inconvenient or unsafe for them to be. See how many ways you can come up with to discourage blacks from voting without enacting an express prohibition of black voting.
Because that is exactly what happened. As the Court pointed out in South Carolina v. Katzenbach (1966):
"Meanwhile, beginning in 1890, the States of Alabama, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Virginia enacted tests still in use which were specifically designed to prevent Negroes from voting. Typically, they made the ability to read and write a registration qualification and also required completion of a registration form. These laws were based on the fact that as of 1890 in each of the named States, more than two-thirds fo the adult Negroes were illiterate while less than one-quarter of the adult whites were unable to read or write. At the same time, alternate tests were prescribed in all of the named States to assure that white illiterates would not be deprived of the franchise. These included grandfather clauses, property qualifications, 'good character' tests, and the requirement that registrants 'understand' or 'interpret' certain matter."
The professionals who drafted the Fifteenth Amendment knew that some of the states would attempt to evade the Fifteenth Amendment even if they could not foresee precisely how the evasions might be designed. And that's why we have section 2, which authorizes Congress to enact "appropriate" legislation to enforce the amendment.