The literacy tests did not in any way interfere with Congress' ability to prevent racial tests from being instituted, which under the 15th, was the only thing they were allowed to even concern themselves with.The "literacy tests" were one of the "racial tests" that had been instituted. In March, 1965, before the Voting Rights Act was implemented, only 6.7% of black adults were registered to vote in Mississippi. By November, 1988, that 6.7% figure had increased to 74.2%.
A literacy test is not a racial test, regardless of the correlation between race and literacy. And the percentage of registered voters among blacks is no measure of the extent to which the 15th amendment is being observed.