I am not saying that all literacy tests are necessarily racial tests. All that I am saying is that Congress is empowered by Section 2 to enforce the Fifteenth Amendment by preventing a state from having a racial test for voting even though the state chooses to call it a literacy test. When, in March, 1965, 6.7% of adult blacks were registered to vote, 69.9% of adult whites were registered to vote.
How could you even begin to explain a disparity like that without considering the possibility that the state of Mississippi was violating the Fifteenth Amendment by using race as an important criteria in determining who was allowed to vote?
If, instead of Congress, the Constitution had empowered you to enforce the Fifteenth Amendment, what would you have done in response to those statistics?
Well, then I may be misunderstanding you. Are you saying that these weren't genuine literacy tests? Because if the election officials were in fact using race, not literacy, as a criterion, then federal authorities would have had the power to act, even without Congress passing a new law. It's called investigating, determining probable cause, making charges, conducting judicial proceedings, etc., you know, that whole "due process" stuff.