> A black southerner of voting age was usually either a former slave, or the child of a former slave, and slaves did not have access to basic education. So a literacy test was, at that time, a racially biased test.

That wasn't the whole problem. The literacy tests were only administered to black voters in the first place because of a "grandfather clase"--anyone whose father and grandfather both had the right to vote had the right to vote automatically, and since the literacy tests were only implemented at the end of slavery, in practical terms they only applied to the black population. Naturally, since the test itself wouldn't disenfranchise whites (except for immigrants, but fuck them too) they were free to make it as difficult as possible.