The 24th amendment prohibits poll taxes in federal elections. I believe, but am not 100%, that the supreme court struck them down in state and local elections using the equal protection clause.
It wasn't the tax but how it was used to disenfranchise blacks. Don't kid yourself the people in power werent about to disenfranchise poor, ignorant whites. There were often exceptions that allowed poor whites to vote without paying. It was the poor whites whose position in society was most threatened by equal rights for blacks. The solid south was maintained largely by gining up fear in the minds of poor ignorant whites.